Progesterone inhibition of estrogen-induced maternal behavior in hysterectomized-ovariectomized virgin rats.

Hysterectomized-ovariectomized virgin rats were tested for maternal behavior following treatment with 100 μg/kg EB immediately at surgery and either oil, 0.5 or 5.0 mg progesterone either 0, 24 or 44 hr following surgery. Stimulus pups were presented 48 hr postoperatively which is counted as Day 0 of testing. EB + oil-treated females displayed short-latency maternal behavior beginning on Day 0. The injection of 5.0 mg progesterone at 0, 24, or 44 hr significantly inhibited the onset of maternal care while the effect of the lower dose of progesterone depended upon the timing of its administration in relation to that of EB. At a dose of 0.5 mg, progesterone given 24 hr following EB, inhibited the appearance of maternal behavior but had no effect given at 44 hr, and resulted in only a partial delay when given at the same time as the EB. Possible mechanisms by which progesterone interfered with the display of maternal behavior were discussed.     

Estrogen and progesterone interactions influencing sexual and social behavior in the brown lemming, Lemmus trimucronatus.

The effects of estrogen and progesterone on the social and sexual behavior of brown lemmings, Lemmus trimucronatus, were investigated. The behavior of hormone-treated and untreated ovariectomized females and sexually vigorous males was observed in six consecutive daily 5-min dyadic encounters. Sexual receptivity, as measured by lordosis, and other social behaviors including nasonasal contact, boxing postures, allogrooming, perineal investigation, and male mounting increased following 48 hr of exposure to daily injections of 0.5 μg estradiol benzoate (EB). Lordosis in EB-primed females was not facilitated or inhibited by short-term (4 hr) exposure to 0.5 mg progesterone (P). Long-term (greater than 24 hr) exposure to P apparently inhibited lordosis and other social behaviors in EB-treated females, although males continued to attempt to mount these females. In EB-treated females a dramatic increase in threat-leaps, directed by the female toward the male, was observed within 4 hr of P injection. Threat-leaps declined when P was withdrawn. Threat-leaps were also observed in ovariectomized females after prolonged exposure to P only (0.5 mg/day). Vaginal perforation and cornification were first apparent 48 hr after EB injection. P-alone treated ovariectomized females also showed vaginal perforation but cornified cells were infrequent and these animals did not show lordosis.     

Duration of estrogen stimulation and progesterone inhibition of maternal behavior in pregnancy-terminated rats

The duration of the effectiveness of estradiol benzoate (EB) on the latency to the onset of maternal behavior was measured in 16-day pregnant rats that were hysterectomized-ovariectomized (HO). Eight groups of HO animals were treated with either a single SC injection of 5 μg/kg of EB or oil at surgery and were initially presented with foster pups at either 24, 48, 72, or 96 hr postoperatively. Compared to their respective controls, EB-treated animals showed singificantly shorter latencies when testing began at 48 and 72 hr but not 24 or 96 hr. In the second experiment, 16-day HO rats were treated with 5 μg/kg of EB at surgery and either oil or 0.5 mg of progesterone at 0, 24, or 44 hr postoperatively. Additional groups received either progesterone or oil at surgery (instead of EB) and a second injection of oil 44 hr later. Testing began 48 hr following surgery for all groups, and the results showed that only the groups injected with EB alone or EB plus progesterone at 44 hr displayed short-latency maternal behavior. It was concluded that a significant reduction in the latency to the onset of maternal behavior can be obtained between 24 and 72 hr after EB treatment and that progesterone when injected concurrently or 24 hr later can inhibit the effectiveness of EB.     

Peripheral plasma progesterone during egg transport in the rabbit.

Peripheral plasma progesterone concentrations were measured in New Zealand rabbits every 6 hr beginning 12 hr before and continuing until 96 hr after either natural mating, hCG injection, or saline injection. The number of ovulation points in naturally mated animals (9.3 +/- 0.6, mean +/- SE) was not significantly different from that in hCG-injected animals (8.6 +/- 1.5). There was a surge in progesterone secretion following both mating and hCG injection. Plasma progesterone concentrations reached a peak prior to ovulation and then fell to basal levels at the time of ovulation. Beginning at approximately 30 hr after the ovulation-inducing stimulus, there was a progressive, significant (P less than 0.001) increase in plasma progesterone concentration, which continued for the duration of the sampling period. The initiation of the postovulatory increase in progesterone secretion corresponds temporally with the movement of eggs from the ampullary-isthmic junction into the isthmus. The progressive increase in plasma progesterone between 30 and 72 hr after the induction of ovulation corresponds with the gradual movement of eggs through the isthmus into the uterus. The data suggest that movement of eggs through the oviductal isthmus is influenced by the postovulatory secretion of progesterone.     

Effect of PGE1 on PGF2 alpha-induced luteolysis in nonbred ewes

Fifty-six ewes were used to study the effects of PGE1 or PGE2 plus PGF2 alpha given into the perivascular space of the ovarian vascular pedicle on luteal function of nonbred ewes. All ewes receiving PGF2 alpha had reduced levels of plasma progesterone and unoccupied receptors for LH at 24 hr after treatment regardless even if they received PGE1 or PGE2 concomitantly. Levels of plasma progesterone in ewes receiving only PGF2 apha were reduced further at 48 hour. Plasma progesterone and unoccupied receptors for LH of ewes receiving PGE2 + PGF2 alpha were maintained at 48 hr at levels seen at 24 hr after treatment, while progesterone in ewes receiving PGE1 + PGF2 alpha at 48 hr returned to levels seen in controls at 48 hr and unoccupied receptors for LH were three fold greater than controls.     

Adrenal and white cell count responses to chronic stress in gestating and postpartum females of the viviparous skink Egernia whitii (Scincidae)

This study investigates the relationships between plasma corticosterone concentrations and white cell counts in captive females of the viviparous lizard Egernia whitii during two phases of the reproductive cycle. Gestating and postpartum females were captured in the field and held in the laboratory for 4 weeks. Plasma corticosterone and progesterone concentrations and white blood cell counts were examined in blood samples taken at capture and after 24 h, 1 week, and 4 weeks in captivity. At 24 h after capture, plasma corticosterone concentrations in both groups had increased significantly compared with initial values but then returned to initial concentrations after 1 week in captivity and remained low in the 4 week samples. Plasma progesterone concentrations remained elevated in the gestating females until the week 4 sample, just prior to parturition. The hormone data suggest that capture and captivity did not represent a significant long-term stressor to these animals. The increase in plasma corticosterone concentration was associated with heterophilia in the differential leucocyte count in both groups of females. Lymphocyte numbers decreased only in gestating females, suggesting that reproductive status may influence the interaction between adrenal activity and immune function.     

Stimulation of maternal responsiveness after pregnancy termination in rats: Effect of time of onset of behavioral testing

To assess the effects of varying the time interval between surgical pregnancy termination and onset of behavioral testing on the expression of maternal behavior, both responsiveness to foster young and nest building-were measured in rats ovariectomized (O), hysterectomized (H), ovariectomized and hysterectomized (OH), or sham-operated (I) on Day 17 of pregnancy at 10:00 and first tested for maternal responsiveness 6, 24, or 48 hr after surgery. When behavioral testing was started 6 hr after surgery O and OH groups responded maternally to foster pups faster than H or I groups, and H females responded maternally faster than I animals (O = OH < H < I). In animals first tested 24 hr after surgery shorter latencies to retrieve and group foster young and nest build were found in the three pregnancy-terminated groups than in I animals, while the three pregnancy-terminated groups did not differ from one another with respect to either behavioral measure (O = OH = H < I). In contrast, when testing was initiated 48 hr after surgery, hysterectomized (H) animals responded maternally faster than did ovariectomized (O and OH) or intact pregnant (I) groups (H < OH < I, H < O = I). These data demonstrate that varying the time of onset of behavioral testing after surgical pregnancy termination affects elicitation of responsiveness to foster young and also affects nest building behavior in pregnancy-terminated animals. The differences in onset of maternal behaviors among pregnancy-terminated animals are discussed in relation to progesterone and estrogen secretion after surgery.     

Periovulatory time courses of plasma estradiol and progesterone in the Japanese monkey (Macaca fuscata)

Periovulatory time courses of plasma estradiol and progesterone were determined in 21 menstrual cycles of 20 Japanese monkeys. Both steroids were measured by radioimmunoassay. Ovulation was detected by serial laparoscopic observations of the ovaries. Three of the 21 cycles were anovulatory cycles. In the remaining 18 ovulatory cycles, a preovulatory estradiol peak occurred on day 12.2±1.4 (range 10–15) of the menstrual cycle. The estradiol concentration at the peak was 431±199 (range 210–930) pg/ml. The time interval between the estradiol peak and ovulation was within 48 hr; the shortest interval was 10–13 hr and the longest 32–48 hr. Although the progesterone levels began to increase slightly (0.6–1.4 ng/ml) before ovulation, they did not show a continuous increase but decreased once before ovulation. The increase in progesterone with development of the corpus luteum after ovulation was very gradual during the first 2 days after ovulation. Subsequently, in 13 of 18 ovulatory cycles the progesterone levels rose rapidly and reached a maximum, 4.0±1.2 (range 2.3–5.7) ng/ml, 4–8 days after ovulation. In 5 of the 18 cycles, the progesterone levels did not rise at all or did not exceed 2.0 ng/ml even if they showed more or less an increase. In the 5 cycles, the length of the luteal phase was 8.2±1.6 (range 6–10) days, which was significantly shorter than that of the former 13 cycles with 14.0±1.1 (range 13–16) days.     

Feedback regulation of gonadotropic hormone secretion in neonatal pigs

The effect of castration and of administration of charcoal-treated porcine follicular fluid (pFF) containing inhibin-like activity on plasma concentration of gonadotropic hormones was studied in neonatal pigs. Plasma follicle-stimulating hormone (FSH) concentration averaged 25.1 +/- 1.5 ng/ml (mean +/- SEM) in 1-wk-old females and gradually declined to 20.2 +/- 0.7 ng/ml 6 wk later. Ovariectomy did not significantly influence plasma FSH concentration. In males, concentration averaged 8.0 +/- 0.7 ng/ml before castration but rose significantly within 2 days after castration. Injection of luteinizing hormone-releasing hormone (LHRH) did not influence plasma FSH concentrations in intact males, but did in females and in 7-wk-old males castrated at 1 wk. Plasma luteinizing hormone (LH) concentrations in 1-wk-old females (2.2 +/- 0.4 ng/ml) gradually declined and were not influenced by castration. Concentrations of plasma LH in 1-wk-old male piglets (2.8 +/- 0.7 ng/ml) were not significantly influenced by castration within 2 days but were significantly higher 6 wk later. LHRH induced a significant rise in plasma LH concentrations in all animals. Injection of pFF resulted in a decline of plasma FSH concentrations in intact and castrated males and in intact females, but did not influence plasma LH concentrations. These data demonstrate a sex-specific difference in the control of plasma FSH, but not in plasma LH concentration in the neonatal pig. Plasma FSH concentrations, but not plasma LH concentrations, are suppressed by testicular hormones in 1-wk-old piglets. Plasma FSH concentrations can be suppressed in both neonatal male and female pigs by injections of pFF.     

Hormonal changes during forced moult induced by progesterone in domestic hen

Forced moulting has been induced in domestic hens by progesterone treatment (5 mg/day) for 25 days. Moult happened between the 11th and 19th day after the first treatment. Endocrine changes were followed during the moult by blood sampling in one week intervals. At the time of the last sampling, new egg laying cycle was initiated in all birds. Plasma progesterone concentration increased significantly in response to the treatment then tended to decrease. Oestrone and testosterone levels were the lowest during the period when feather loss was most intensive and increased in the course of feathering. This increase was significant in the case of oestrone. The level of 17-beta-oestradiol did not vary during moult induced by progesterone treatment. Plasma concentration of thyroxine significantly increased during feather loss, showing a maximum in the second and/or third week after the beginning of the treatment, while it decreased when feather growth had begun. Plasma triiodothyronine as well as corticosterone levels were the highest during the latest phase of moult, at the time of feather outgrowing. It has been supposed that moulting would be initiated in response to the synergistic effect on feather follicles of progesterone and thyroxine, which was stimulated by the progesterone treatment. The atrophic stage of the ovary suggested that progesterone was probably of adrenal origin. It was assumed that triiodothyronine and oestrone were responsible for controlling feather outgrowth.     

Route of prostaglandin F2alpha injection and luteolysis in mares (38519).

Nine groups of pony mares (3/group) were used in a 3 times 3 factorial experiment. The factors were dose of PGF-2 alpha (0, 0.25 of 1.25 mg and route of administration (im, iu or il). Mares were laparotomized and treated on day 7 postovulation. Jugular blood was collected for progesterone RIA at 0 (pretreatment) and 1,6,12,24,48, and 72 hr posttreatment. In mares given either 0.25 mg or 1.25 mg PGF-2alpha, progesterone concentrations were not significantly different among the three routes at any of the posttreatment times studied except at 6 hr posttreatment. In mares given 0.25 mg, progesterone concentrations at 6 hr was less (p less than 0.05) for mares injected im than for mares injected iu. Compared to pretreatment progesterone values, PGF2-alpha (0.25 mg and 1.25 mg groups combined) administration significantly decreased progesterone concentration by 12 hr posttreatment in mares injected im and 24 hr in mares injected iu or il. In the iu group, a significant increase in progesterone concentration occurred between 1 and 6 hr followed by a significant decrease at 12 hr posttreatment. There were no significant differences among the three routes for intervals from treatment to estrus or ovulation, length of posttreatment estrus or length of interovulatory interval. Injection of either 0.25 mg or 1.25 mg PGF-2alpha significantly shortened the interval from treatment to estrus. Although 0.25 mg tended to shorten the interval from treatment to ovulation and interovulatory interval, these two end points were significantly shortened only in mares given 1.25 mg PGF-2alpha. Results indicated that local administration (iu or il) did not improve the luteolytic efficacy of PGF-2alpha over systemic administration (im).     

Progesterone concentrations and contents in the plasma, ovary, adrenal gland and placenta of the pregnant Natal clinging bat Miniopterus schreibersii natalensis

Reproduction in the Natal clinging bat in South Africa at 25°S is characterized by fertilization in April, implantation during July and parturition in November. In the present study plasma progesterone concentrations peaked two months before implantation, this coinciding with significant peaks in ovarian progesterone content and concentration. Plasma progesterone concentrations remained relatively low during and after implantation, but increased significantly during the limb bud stage to peak levels one month before parturition. Adrenal progesterone content and concentration did not follow a specific trend; post-implantation ovarian progesterone content and concentration remained at baseline values. Placental progesterone content increased significantly with foetal weight (age). The present study showed that pregnancy in these bats is characterized by two major sources of progesterone, with the ovary as the primary source before implantation and the placenta the primary source during embryonic and foetal development.     

Duration of hormonal deprivation: influences on physiological and behavioral responsiveness to estradiol

A total of 54 ovariectomized female guinea pigs were divided into three groups and tested six times at 2-week intervals for their responsiveness to exogenous ovarian hormones (3 days of 4 micrograms/kg estradiol benzoate plus 1 day of 0.4 mg/kg progesterone) or control injections (0.2 ml oil vehicle). Two weeks after ovariectomy, treatment with estradiol significantly reduced food intake and body weight, and also produced vaginal membrane rupture in 98.1% of the females. When tested for sexual behavior at 4, 6, and 8 hr after the progesterone injection, 29 of the subjects (53.7%) displayed lordosis in response to manual stimulation. Twelve weeks after ovariectomy, the effects of estradiol on food intake, body weight, and vaginal membrane condition had not diminished. However, the overall proportion of females from which lordosis could be elicited declined to 27.8%. Biweekly injections of estradiol benzoate plus progesterone to one of the groups of females did not prevent this decline in the sexual response. Based on these results, it was concluded that the observed reduction in behavioral lordosis does not represent a general decline in the responsiveness of ovariectomized guinea pigs to estrogenic stimulation, but may involve changes in their responsiveness to progesterone or in other mechanisms more specifically associated with sexual behavior.     

Effect of prostaglandin F2 alpha (PGF2 alpha) on sources of progesterone and pregnancy in intact, ovariectomized and hysterectomized 90-100 day pregnant ewes.

A single dose of 8 or 16 mg of PGF2 alpha per 58 kg body weight was injected intramuscular into intact, ovariectomized or hysterectomized 90-100 day pregnant sheep in three separate experiments. Both doses of PGF2 alpha decreased the weights of the corpora lutea (P less than or equal to 0.05) and the concentration of progesterone in ovarian venous plasma at 72 hr (P less than or equal to 0.05) compared to the 0 hr sample within treatment groups and to control ewes at 72 hr in intact and hysterectomized pregnant ewes. In hysterectomized pregnant ewes, progesterone in jugular plasma declined (P less than or equal to 0.05) from 0 to 72 hr but never fell below 4 mg/ml and this decrease in progesterone after 8 or 16 mg PGF2 alpha was greater than in control hysterectomized ewes (P less than or equal to 0.05). There was a significant decrease in progesterone over time in jugular or uterine venous plasma in the presence of absence of the ovaries in 90-100 day pregnant ewes (P less than or equal to 0.05) but the profiles of progesterone were not different between vehicle and PGF2 alpha-treated ewes (P greater than or equal to 0.05). Uterine venous progesterone never declined below 30 ng/ml in the presence or absence of the ovaries and there was a significant quadratic increase (P less than or equal to 0.05) in uterine venous progesterone toward the end of the 72 hr sampling period indicating an increase in steroidogenic activity of the placenta. PGF2 alpha did not affect the number of abortions in intact or ovariectomized pregnant ewes (P greater than 0.05). Thus, the corpus luteum of sheep at 90-100 days of pregnancy is functional and responsive to PGF2 alpha, placentomes are functional but do not appear to be responsive to the doses of PGF2 alpha tested and PGF2 alpha was not an abortifacient over the 72 hr treatment period.     

Growth hormone and seawater adaptation in coho salmon, Oncorhynchus kisutch

1. Effects of growth hormone (GH) were examined on short-term aspects of seawater adaptation in coho salmon smolts. 2. Injection of somatostatin (SRIF) immediately prior to seawater entry suppressed plasma GH levels, but did not have any significant effects at 6 or 12 hr on hematocrits, plasma glucose or plasma Na+ levels. 3. Plasma GH levels increased 250% within 36 hr after seawater exposure. 4. Plasma glucose levels, in contrast, were significantly lower in the seawater fish after 36 hr post-exposure. 5. Plasma Na+ levels increased to 190 mEq/1 by 24 hr but subsequently returned to freshwater levels while hematocrits showed no significant changes over the 72 hr of exposure. 6. The significance of these results is discussed in terms of successful seawater adaptation in coho salmon.     

Effects of estradiol benzoate and progesterone on superoxide dismutase activity in the thymus of rats

The activity of mitochondrial superoxide dismutase (MnSOD) and cytosol superoxide dismutase (CuZnSOD) was measured in corresponding subcellular fractions prepared from the thymi of intact and chronically gonadectomized (GX) rats of both sexes, as well as of GX male and female rats injected subcutaneously with a single dose of 5 microg estradiol benzoate (EB) and/or 2 mg progesterone (P). Animals were sacrificed 2 h or 24 h following hormone treatment. In the females, the activity of MnSOD in the thymus was stable during the estrous cycle and did not change after ovariectomy. Treatment of GX females with estradiol benzoate resulted 2 h later in a significant elevation of MnSOD activity, whereas 24 h later the activity returned back to control values. On the other hand, treatment of GX females with progesterone had no effect on the MnSOD activity. However, combined hormone treatment, in which EB injection preceded progesterone injection by one hour, enhanced the effect on MnSOD activity similar to that of estradiol benzoate alone. The activity of CuZnSOD in cycling rats was increased in proestrus, whereas removal of the ovaries kept the values at low diestrus and estrus levels. Contrary to MnSOD, CuZnSOD activity did not change after EB treatment of GX females, while progesterone increased the enzyme activity at 2 h and 24 h after hormone treatment. However, combined EB+P treatment proved to be ineffective. In the males, neither MnSOD nor CuZnSOD activity was affected by the removal of testes or by progesterone treatment of GX animals. Only EB injection to GX rats significantly increased CuZnSOD activity 24 h later.     

Postmortem biochemical indices of antemortem hemorrhagic shock

The purpose of this study was to determine if the perturbations in two glycolytic metabolites that occur during hemorrhagic shock can be used as discriminatory postmortem indicators of death resulting from severe hemorrhagic shock. Two groups of male albino Sprague-Dawley rats were hemorrhaged by withdrawing either 40% (Group I) or 45% (Group II) of the total blood volume. Glycogen and lactate concentrations were determined at 0 and 48 hr postmortem in the following tissues and organs: diaphragm, heart, liver, kidney cortex, and kidney medulla. The differences in lactate and glycogen in Group I at 0 hr were not significantly different from the nonhemorrhaged controls, with the exception of the lower liver glycogen concentration (58% of control). In Group II glycogen concentration was significantly reduced at 0 hr in the diaphragm (70% of control), liver (37%), and kidney medulla (55%). Lactate concentration was higher in all tissues examined by 270-640%; within 48 hr all tissues for both control and hemorrhaged animals had declined to baseline levels of glycogen concentration, whereas lactate levels had increased as much as 34-fold. There were no highly significant differences in glycogen at 48 hr between the control and hemorrhaged groups. In Group II the lactates were similar for both the control and hemorrhaged animals with the exception of the higher concentrations in the kidney cortex (54%) and medulla (41%). It was concluded from these findings that although significant metabolic perturbations are present at the time of death due to hemorrhage these differences do not persist up to 48 hr postmortem, with the possible exception of the kidney lactate concentrations.     

Conception, embryonic development and corpus luteum function in beef cattle open for two consecutive breeding seasons

High-fertility (control cows) and low-fertility (cows and heifers not pregnant after two consecutive breeding seasons — twice-open) cyclic bovine females were treated with a single injection of 1000 IU of human chrionic gonadotropin (HCG) or 100 μg of gonadotropin releasing hormone (GnRH) to enhance and/or hasten corpus luteum formation and progesterone secretion, and improve conception rate in the low-fertility females. Hormone treatments were administered to 38 parous control cows, 34 twice-open parous cows, and 27 twice-open nonparous heifers immediately after natural mating by a fertile bull. Blood samples were collected on Days 3, 6, 9, 12, and 18 after mating for determination of systemic progesterone concentrations. Pregnancy rate at necrospy approximately 33 days after mating (range 31–37) was higher in control cows (73.0%) than in twice-open cows (48.4%; P<0.05) or twice-open heifers (34.6%; P<0.01). Pregnancy rate was not affected by the HCG or GnRH treatment. The HCG treatment increased plasma progesterone concentrations in twice-open heifers but not in control or twice-open cows. Progesterone was unaffected by the GnRH treatment. Systemic progesterone concentrations were higher in control than in twice-open females but did not differ between pregnant and nonpregnant females of Days 3, 6, 9 and 12 after mating. Enhanced gonadotropin stimulation at estrus by injection of either HCG or GnRH did not increase pregnancy rate or systemic progesterone concentrations (except in HCG-treated twice-open heifers) in low- or high-fertility females. Lower pregnancy rates in twice-open females were not associated directly with the lower systemic progesterone concentrations.     

Does corpus luteum function autonomously during estrous cycle in the rat? A possible involvement of LH and prolactin

This study examined the of LH and prolactin in the control of corpus luteum function during 4-day cycles in the rat. Bromocriptine (BRC) treatment was performed on proestrus or/and estrus morning that means before or after the preovulatory release of LH. This caused complete blood prolactin depression from the time of injection until diestrus 1 afternoon. This decrease in blood prolactin concentration was associated with a rise in the tonic level of LH secretion in those females which received BRC as soon as on proestrus. We first observed that injection on the morning of proestrus of doses of BRC capable of blunting prolactin secretion on proestrus afternoon did not significantly impair the preovulatory release of LH and did not prevent ovulation occurring during the following night. The life span of the corpora lutea edified from ovarian follicles rupturing before or under BRC administration did not exceed that of those formed under physiological circumstances since 4-day cycles culminating in ovulation constantly took place in all the treated animals whatever the time of BRC injection. To determine the pattern of luteal activity in the absence of prolactin secretion, we measured blood progesterone concentration from estrus until late diestrus in female rats injected with BRC on proestrus and/or estrus at 1100 h. The initiation of the function of corpus luteum on estrus and the achievement of its full activity on diestrus 1 did not appear to be affected by BRC. By contrast the level of blood progesterone declined more rapidly on the morning of diestrus 2 in BRC-treated than in control females. The capacity for autonomous progesterone secretion by corpus luteum of the cycle was discussed in the light of previous and present observations.     

Evidence from plasma progesterone concentrations for male-induced ovulation in the brush-tailed bettong, Bettongia penicillata.

Female brush-tailed bettongs, Bettongia penicillata, were housed with either an intact or vasectomized male or isolated from males in the peripartum period. Development of the quiescent corpus luteum formed at the post partum oestrus was initiated by removing the pouch young. Blood samples for analysis of plasma progesterone were collected from the females 2 days before removal of pouch young, daily for 5 or 6 days and then 2-3 times each week until 19 days after removal of pouch young. Plasma progesterone profiles were similar in pregnant and nonpregnant cycles. There was an early progesterone peak (1206 +/- 121 pg ml-1, mean +/- SEM; n = 16) between days 2 and 5 after removal of pouch young, and a second period of high concentrations (greater than 800 pg ml-1) before birth on day 17.4 +/- 0.2 (n = 16). The interval between the early peak and birth was 14 or 15 days. On five of 34 occasions, no increases in plasma progesterone concentrations occurred after removal of pouch young. On 12 of 15 occasions for 13 females that had been isolated from males post partum, plasma progesterone concentrations also remained low (less than 100 pg ml-1) and did not change after removal of pouch young. Females that showed no increases in plasma progesterone concentration after removal of pouch young had significantly lower (P less than 0.001) plasma progesterone concentrations while lactating than those females that did undergo a cycle after removal of pouch young (60 +/- 4 pg ml-1, n = 17 and 225 +/- 23 pg ml-1, n = 30, respectively). Females isolated from males post partum, and monitored until day 12 after removal of the pouch young, and that showed no increases in progesterone in this period, had ovaries that contained no corpus luteum, only corpora albicantia and numerous atretic or developing follicles. We conclude that brush-tailed bettongs are induced ovulators, a characteristic described for only one other marsupial, Monodelphis domestica, from South America.