Effects of the opioid analgesic oxymorphone hydrochloride on reproductive function in male and female rats

BACKGROUND: Endogenous opioids seem to regulate hypothalamic gonadotropin release in both males and females, as evidenced by the effects of opioid agonists and antagonists on LHRH release and reproductive hormone levels. The effects of long‐term oral administration of opioid analgesics on reproductive function have not been well characterized. METHODS: The reproductive effects of oxymorphone, a potent opioid agonist, were investigated in male and female Crl:CD(SD) IGS BR rats at oral doses of 0, 5, 10, and 25 mg/kg/day (25 animals/sex/group). Males were treated for approximately 9 weeks (mated after 4 weeks of dosing). Females were treated for 14 days before mating, and through Gestation Day (GD) 7. Estrous cycling was evaluated during the premating period. On GD15, pregnancy status and the numbers of corpora lutea, implantation sites, live and dead embryos were determined. Epididymal and testicular sperm counts and epididymal sperm motility and morphology were evaluated in males. RESULTS: Two males given 25 mg/kg/day died. Behavioral changes and deficits in body weight gain occurred at all doses. There were no effects of oxymorphone on reproductive function or sperm parameters in males. The estrous cycle was prolonged in females given 25 mg/kg/day (mean of 5.3 vs. 4.3 days in controls). A small, but consistent decrease in the numbers of corpora lutea (with associated decreases in implantation sites and embryos) occurred in females given ≥10 mg/kg/day. There were no effects on mating or fertility in females. CONCLUSIONS: Oxymorphone seems to partially inhibit ovulation in female rats, with no significant effects on male reproductive outcome. Birth Defects Res (Part B) © 2007 Wiley‐Liss, Inc.     

The Effect of Atrazine Administered by Gavage or in Diet on the LH Surge and Reproductive Performance in Intact Female Sprague‐Dawley and Long Evans Rats

Atrazine (ATR) blunts the hormone‐induced luteinizing hormone (LH) surge, when administered by gavage (50–100 mg/kg/day for 4 days), in ovariectomized rats. In this study, we determined if comparable doses delivered either by gavage (bolus dose) or distributed in diet would reduce the LH surge and subsequently affect fertility in the intact female rat. ATR was administered daily to intact female Sprague‐Dawley (SD) or Long Evans (LE) rats by gavage (0, 0.75 1.5, 3, 6, 10, 12, 50, or 100 mg/kg/day) or diet (0, 30, 100, 160, 500, 660, or 1460 ppm) during one complete 4‐day estrous cycle, starting on day of estrus. Estrous status, corpora lutea, ova, and LH plasma concentrations were evaluated. A second cohort of animals was mated on the fourth treatment day. Fertility metrics were assessed on gestational day 20. A higher portion of LE rats had asynchronous estrous cycles when compared to SD rats both during pretreatment and in response to ATR (≥50 mg/kg). In contrast, bolus doses of ATR (≥50 mg/kg) inhibited the peak and area under the curve for the preovulatory LH surge in SD but not LE animals. Likewise, only bolus‐treated SD, not LE, rats displayed reduced mean number of corpora lutea and ova. There were no effects of ATR administered by gavage on mating, gravid number, or fetus number. Dietary administration had no effect on any reproductive parameter measured. These findings indicate that short duration, high‐bolus doses of ATR can inhibit the LH surge and reduce the number of follicles ovulated; however, dietary administration has no effect on any endocrine or reproductive outcomes     

Low P50 in deer mice native to high altitude

Whereas it is widely believed that animals native to high altitude show lower O2 partial pressures at 50% hemoglobin saturation (P50) than do related animals native to low altitude, that "fact" has not been well documented. Consequently, P50 at pH 7.4, PCO2(7.4), the CO2 Bohr effect, and the buffer slope (delta log PCO2/delta pH) were determined via the mixing technique in Peromyscus maniculatus native to a range of altitudes but acclimated to 340 or 3,800 m. PCO2(7.4) and buffer slope were substantially lower at high altitude. The change in P50(7.4) between acclimation altitudes was minimal (0.8% increase at 3,800 m), because of counterbalancing changes in PCO2, 2,3-diphospho-D-glycerate concentration, and perhaps other factors. At both acclimation altitudes there was a highly significant negative correlation between P50(7.4) and native altitude. Since pH in vivo probably increases slightly at high altitude, the data on P50 corrected to pH 7.4 are probably underestimates of the difference in in vivo P50 at low vs. high altitude. Hence these results corroborate theoretical predictions that low P50 is advantageous under severe hypoxic stress.     

Effects of larval tapeworm (Taenia taeniaeformis) infection on reproductive functions in male and female host rats

This report examined the effects of larval tapeworm infection on the reproductive functions in both male and female host rats. Female rats were matched by age, then randomly assigned to control and treatment groups (infected with larval tapeworms). Estrous cycles were determined by vaginal smear with 95% of the control group exhibiting 4-day normal cyclicity and only 55% of the treated group exhibiting normal cycles. Female fertility was then evaluated for the normally cycling rats based on the percentage of successful matings on the evening of proestrus, number of implantation sites on Day 8 of pregnancy, and number of pups born at term. The normally cycling rats exhibited 96% successful mating, 12.95 +/- 1.80 implantation sites, and 11.20 +/- 1.80 pups born. Five months after larval tapeworm infection, the fertility parameters were decreased to 79%, 9.10 +/- 1.20, and 7.50 +/- 1.50, respectively. The control females were then used in a study of male fertility after larval tapeworm infection employing the same parameters used to test female fertility. At the onset of the study, control groups exhibited 95% successful mating, 12.50 +/- 1.50 implantation sites, and 11.60 +/- 1.60 pups born at full term. After the 5-month infection period, the parameters were substantially reduced to 29%, 6.20 +/- 0.80 implantation sites, and 5.10 +/- 0.80 pups, respectively. Average testosterone concentrations in serum and testis from control male rats were 8.80 +/- 0.95 ng/ml and 3.88 +/- 0.25 ng/mg protein, respectively. After the 5-month infection period, these levels were reduced to 2.47 +/- 0.31 ng/ml and 1.28 +/- 0.12 ng/mg protein, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effect of restraint stress in early pregnancy in mice

Mice were exposed to 5 h of restraint stress on Days 1-3, 4-6, or 1-6 of pregnancy in the morning (08:30-13:30 h, a.m.) or afternoon (13:30-18:30 h, p.m.). Stress reduced the pregnancy rate from 90 to 52% (P less than 0.005) and average litter size on Day 18 from 8.2 to 5.2 young (P less than 0.005). Stress for 6 days was more effective than for 3 days (P less than 0.005) and an a.m. stress was more effective than a p.m. stress (P less than 0.005) in reducing the average litter size. Animals examined on Day 7 after 6 days of a.m. stress had decreased numbers of normal corpora lutea (CL), increased numbers of abnormal CL, decreased serum progesterone concentrations and tended to have fewer implantation sites. Abnormalities of embryo transport and implantation were also present. Changes in CL morphology and embryo transport and development were evident on Day 4 after only 3 days of restraint stress. These results show that many reproductive events of early pregnancy can be disrupted by restraint stress.     

Chronologic and quantitative modalities of luteinization and ovulation in the female rat exposed to FSH action at the beginning of 4-day cycles]

Female Wistar rats were injected with two successive doses of 150 mug FSH on dioestrus I (5 p. m.) and dioestrus II (9 a.m.) of 4-day cycles. Superovulation and formation of an increased number of corpora lutea with included oocytes occurred in these animals. Meprobamate or pentobarbitone injections at 11.30 a.m. et 1.30 p.m. respectively completely blocked ovulation and luteinization phenomena in FSH-treated females. Ovulation normally occurred in FSH-treated females injected with meprobamate at 5.30 p.m. It was concluded that no modification of timing of the 'critical period' resulted from FSH treatment.     

Effect of treatment with recombinant bovine somatotropin on responses to superovulatory treatment in swamp buffalo (Bubalus bubalis)

The objective of this study was to determine the effect of treatment with recombinant bovine somatotropin (rBST) on the response to superovulatory treatment in swamp buffalo. Estrous cycles of 16 buffalo cows were synchronized by intravaginal administration of progesterone and estradiol benzoate, and the cows were then randomly divided into 2 groups. The rBST-treated group received 250 mg of a sustained-release formula of rBST on Day 4 after progesterone implantation, whereas the control group did not receive rBST. Both groups were then given a superovulatory regimen of twice daily injections of FSH for 3.5 d (total dose of 260 mg, i.m.), between Days 9 and 11 after administration of progesterone. The cows were bred naturally 1 d after the last FSH injection, then 6 d after breeding they were slaughtered, and their reproductive tracts were removed. The numbers of corpora lutea (CL) and follicles were recorded, and embryos were flushed out of the uterine horns. There were no significant differences between the rBST-treated and control cows for the mean numbers (+/- SEM) of CL (6.0 +/- 2.2 vs 4.3 +/- 1.1), follicles (15.9 +/- 4.1 vs 19.8 +/- 2.9), or total embryos recovered per collection (4.5 +/- 1.6 vs 2.3 +/- 1.0). However, there were significant differences between rBST-treated and control cows for the numbers of transferable embryos per collection (3.0 +/- 1.0 vs 0.8 +/- 0.3; P < or = 0.05) and the overall proportion of transferable embryos (75 vs 33%; P < or = 0.01). The results of this study show that pretreatment of swamp buffalo with rBST significantly increases the production of transferable embryos in response to superovulation.     

Estrous behavior and ovarian activity in peripuberal heifers.

Estrous behavior and ovarian activity were investigated in peripuberal heifers. Reproductive tracts of 37 Holstein heifers were examined per rectum twice weekly beginning at least 30 days before the 1st ovulation and continuing until 10 days after the 7th ovulation. The signs of 1st estrus occurred at an average of 279 days of age and the 1st corpus luteum was not detected until 30 days later (p .01). The mean interestrual interval was 20 days for the 1st 7 cycles (p .05). The differences in interestrual intervals for cycles accompanied by silent, nonstanding, and standing estrous behavior, and by different ovarian conditions were not significant (p .05). Silent, nonstanding, and standing estrus occurred during 7, 25, and 68% of 245 estrous cycles, respectively. The occurrence of standing estrus increased from the 1st to 7th cycle (p .01). Normal corpora lutea occurred during 61% of 245 cycles, cystic corpora lutea during 14%, anovulaton during 14%, and cystic follicles during 11%. The occurrence of anovulation decreased from 1st to 7th estrus while the occurrence of corpora lutea increased during the same period (p .01).     

Adverse effects on female development and reproduction in CD-1 mice following neonatal exposure to the phytoestrogen genistein at environmentally relevant doses

Outbred female CD-1 mice were treated with genistein (Gen), the primary phytoestrogen in soy, by s.c. injections on Neonatal Days 1-5 at doses of 0.5, 5, or 50 mg/kg per day (Gen-0.5, Gen-5, and Gen-50). The day of vaginal opening was observed in mice treated with Gen and compared with controls, and although there were some differences, they were not statistically significant. Gen-treated mice had prolonged estrous cycles with a dose- and age-related increase in severity of abnormal cycles. Females treated with Gen-0.5 or Gen-5 bred to control males at 2, 4, and 6 mo showed statistically significant decreases in the number of live pups over time with increasing dose; at 6 mo, 60% of the females in the Gen-0.5 group and 40% in the Gen-5 group delivered live pups compared with 100% of controls. Mice treated with Gen-50 did not deliver live pups. At 2 mo, >60% of the mice treated with Gen-50 were fertile as determined by uterine implantation sites, but pregnancy was not maintained; pregnancy loss was characterized by fewer, smaller implantation sites and increased reabsorptions. Mice treated with lower doses of Gen had increased numbers of corpora lutea compared with controls, while mice treated with the highest dose had decreased numbers; however, superovulation with eCG/hCG yielded similar numbers of oocytes as controls. Serum levels of progesterone, estradiol, and testosterone were similar between Gen-treated and control mice when measured before puberty and during pregnancy. In summary, neonatal treatment with Gen caused abnormal estrous cycles, altered ovarian function, early reproductive senescence, and subfertility/infertility at environmentally relevant doses.     

Functional and morphological changes in the hypothalamo-pituitary-gonadal axis of aged female rats.

Age-related functional and morphological alterations in the hypothalamo-pituitary-gonadal axis were investigated in old recurrently pseudopregnant (RPP) female rats, and these alterations were compared with those in young diestrous rats. LHRH in the median eminence (ME) and mediobasal hypothalamus (MBH) as well as plasma FSH, LH, and progesterone were measured by RIA. LHRH in the lateral ME (LME) and pituitary FSH and LH were evaluated by morphometry and densitometrical immunocytochemistry. Furthermore, by light microscopy, we classified and counted the number of ovarian follicles and corpora lutea. LHRH concentrations in the ME and MBH were similar in old and young rats, whereas in old rats, plasma FSH was markedly increased, LH was moderately increased, and plasma progesterone was unchanged. The number and the total area and immunoreactivity of LHRH-labeled axon cross sections in the LME were reduced in old rats. The number of nucleated FSH-labeled cells and total FSH area and immunoreactivity were almost twice in old compared with young animals. The measurements of LH-labeled cells were not different between the two groups. In old rats, the numbers of ovarian follicles and corpora lutea were reduced and that of atretic follicles increased. In conclusion, age-related morphological impairments of LHRH axons associated with an increased number of FSH gonadotropes and higher plasma FSH in our old RPP rats suggest hypothalamic and pituitary disturbances, which may largely contribute to the complex hormonal disarrangement responsible for the decline of reproductive functions in old female rats.     

A decade of rabbit fertility data: study of historical control animals.

Reproductive data for 1795 artificially inseminated Hra: (NZW)SPF control rabbits in 93 developmental toxicity studies conducted from 1980 through 1989 were summarized. Data were obtained during terminal Caesarean-sectioning procedures performed on animals which survived to day 29 of gestation and during postmortem evaluation of does which aborted, delivered prematurely, or were found dead. Significant seasonal variation was not observed. Average pregnancy rate, percentage of rabbits that aborted, and percentage of rabbits that delivered prematurely throughout the decade were 86, 2.0, and 1.6%, respectively. Average numbers of corpora lutea, implantations, live fetuses, dead fetuses, early resorptions, and late resorptions for each doe that survived to scheduled termination were 10.8, 7.8, 7.1, 0.02, 0.49, and 0.15, respectively. The various vehicles used, routes of administration, and a variety of maternal and paternal factors were compared with the fertility data, and no correlations were observed. Rabbits that aborted earlier in gestation had fewer implantations than does which aborted late or delivered prematurely. Does which resorbed 100% of their conceptuses had fewer corpora lutea and implantations when compared with rabbits in the remainder of the population. Rabbits pregnant at scheduled termination which had a low number of corpora lutea or implantation sites had higher than expected pre- and postimplantation losses relative to the population as a whole. Does with a high number of corpora lutea had significantly higher preimplantation loss relative to the general population. This may indicate the presence of a "ceiling value" for the number of ova that can become fertilized and/or implant when the ovulation rate is high and which probably varies according to strain as well as a number of factors related to the individual rabbit. Based upon the results of this study and the work of a previous author, a minimum of four corpora lutea may be necessary for the successful maintenance of pregnancy in the New Zealand white rabbit. The minimum number of corpora lutea required may be strain dependent and may bear a relationship to the normal litter size of the strain.     

Effects of Annona squamosa extract on early pregnancy in rats.

Annona squamosa Linn., family Annonaceae, is said to show varied medicinal effects, including insecticide, antiovulatory and abortifacient. The purpose of present study was to investigate if A. squamosa seed aqueous extract, in doses higher than that popularly used to provoke abortion, interferes with reproductive performance, and to correlate the ingestion of this extract with possible alterations in rat embryonic implantation. Doses of 300 mg/kg (Treated Group I, n = 17) and 600 mg/kg (Treated Group II, n = 12) body wt. were administered by gavage, during days 1 to 5 of pregnancy (preimplantation period). The control group (n = 13) received water in the same manner, during the same period for comparison with experimental groups. The animals were euthanized on day 10 of pregnancy. Treatment of dams during the preimplantation period showed no signs of toxicity, and no alteration in the corpora lutea, implantations and embryo in terms of development numbers. The percentage of preimplantation and postimplantation losses in treated groups I and II did not differ from those of control. Treatment with aqueous extract of A. squamosa seeds caused no morphological change in the endometrium. The absence of morphological alterations in uterine epithelial cells in treated groups I and II permitted a viable embryonic implantation, as verified by the number of embryos in development at day 10 of pregnancy. Thus, A. squamosa seed aqueous extract did not interfere with the reproductive performance of pregnant rats.     

Increase in litter size and decrease of post-implantation loss of fetuses observed in an SPF colony of Wistar-Imamichi rats

The number of delivered offspring, corpora lutea and implantation sites was observed in an SPF colony of Wistar-Imamichi rats bred in a barrier system free of specified microorganisms and parasites and was compared with that of rats bred under conventional conditions. The litter size of 6142 SPF rats ranged from 1 to 20, averaging 12.4 +/- 0.04, a value significantly higher (P less than 0.001) than that of conventionally bred rats (11.0 +/- 0.04). Although the number of corpora lutea was also higher in SPF rats than under conventional conditions, the number of post-implantation losses in SPF rats with less than 10 offspring showed a marked decrease compared with conventionally bred rats. These results show that the number of delivered offspring is higher in SPF rats than in conventionally bred rats and indicate that the increase in litter size is due to the increase in the number of corpora lutea and decrease in post-implantation loss of embryos of fetuses.     

Growth of Qinghai Tibetans living at three different high altitudes

This study compares the stature, weight, skinfolds, upper arm muscle area, and chest dimensions of Tibetan children, adolescents, and young adults who were born and raised, or who had lived from infancy, at 3,200 m, 3,800 m, and 4,300 m in Qinghai Province, People's Republic of China. While the individuals measured in Qinghai are among the tallest and heaviest Tibetans reported in the literature, they are nevertheless smaller and lighter than well-off children living at low altitude. The pattern of size variation among Tibetan males and females measured at the three high altitudes, along with evidence of a secular trend at 4,300 m, suggests that nutrition may significantly effect growth at high altitude. Only minor differences in thorax dimensions exist between Tibetan males and females measured at 3,200 m and 3,800 m. However, Tibetan males at 4,300 m possess slightly narrower and deeper chests (during and after adolescence) than males at 3,200 m and 3,800 m. Since individuals from 3,800 m and 4,300 m belong to the same local populations, this characteristic is unlikely to be genetically determined. However, it may be related to differences in the degree of hypoxia or to the influences of other environmental conditions.     

Effects of photoperiod on the reproductive condition of Nile grass rats (Arvicanthis niloticus) from an equatorial population

We evaluated the effects of photoperiod on the reproductive condition of male and female Nile grass rats (Arvicanthis niloticus) descended from members of an equatorial population trapped 2°S of the equator. Study animals housed in 12:12 light:dark (LD) cycles were transferred either to short photoperiod (9:15) or long photoperiod (15:9) for 9 weeks (males) or 11 weeks (females), and various reproductive parameters were assessed. We observed no differences between short‐ and long‐day males with respect to plasma concentration of testosterone, testicular mass, seminal vesicle mass, or spermatogenesis. Similarly, we observed no differences between short‐ and long‐day females with respect to oestrous cycles, uterine mass, follicle size, or presence of corpora lutea. Reproductive parameters of male and female A. niloticus housed in short‐ and long photoperiods were similar to those typically observed among animals descended from the same equatorial population and housed in LD 12:12. Thus, photoperiod appears not to elicit changes in reproductive condition among A. niloticus from populations whose native habitat lies within 2° of the equator. These data contrast with the results of other studies indicating that photoperiod alters reproductive condition in A. niloticus populations living >10° from the equator.     

Neonatal superior ovarian nerve transection disturbs the cyclic activity of the female rats

The neural pathway most related with ovarian steroidogenesis has been identified as the superior ovarian nerve (SON). This work constitutes the first study of the effects of early ovarian SON transection, which was performed in rats of 4 days of age (SON-t rats) to magnify the effects of the denervation. The rats were studied at the prepubertal (30 days), peripubertal (41 days) and adult cyclic in dioestrus (60 days) reproductive stages. The SON-t rats showed a delay of vaginal opening, a notable disruption of oestrous cyclicity, and a large number of corpora lutea. In all the stages, the circulating levels of FSH, prolactin and growth hormone were lower in SON-t rats than in controls, whereas LH did not vary. Serum androstenedione levels were higher in SON-t rats at 30 days and lower at 41 days, compared with control animals while no difference was observed at 60 days. Serum progesterone levels did not differ between control and SON-t, but serum oestradiol concentrations were higher in SON-t rats in all of the stages. At the peripubertal stage, there were fewer ovarian beta-adrenergic receptors in SON-t ovaries, associated with a rise in the ovarian content of norepinephrine, but no changes were observed in SON-t rats at 30 and 60 days with respect to the controls. The release of progesterone in vitro from luteal cell in SON-t rats at 60 days was reduced in basal condition and under ovine LH or FSH stimulation, when compared with control animals; while no difference was observed in presence of isoproterenol or androstenedione in the culture medium. In corpora lutea of SON-t rats at 60 days, no change was observed in the activity of 3beta-hydroxysteroid dehydrogenase (3beta-HSD), but the activity of 20alpha-hydroxysteroid dehydrogenase (20alpha-HSD) was reduced, suggesting abnormal luteolysis in spite of the large number of corpora lutea. The interruption of innervation at an early age by SON transection is very important in the regulation of ovarian development in prepubertal and cyclic rats. The functional changes observed in the ovary suggest a possible alteration in the hypothalamic-hypophyseal axis.     

Lynx reproduction – Long-lasting life cycle of corpora lutea in a feline species

A review of lynxes’ reproductive biology and comparison between the reproductive cycles of the domestic cat and lynxes is presented. Three of the four lynx species (the bobcat excluded) express quite similar reproductive pattern (age at sexual maturity, estrus and pregnancy length, litter size). Similarly to the domestic cat, the bobcat is polyestric and can have more than one litter per year. Domestic cats and many other felid species are known to express anovulatory, pregnant and pseudo-pregnant reproductive cycles in dependence on ovulation induction and fertilization. The formation of corpora lutea (CLs) occurs after ovulation. In pregnant animals, luteal function ends with parturition, whereas during pseudo-pregnancy a shorter life span and lower hormone secretion are observed. The life cycle of corpora lutea in Eurasian lynxes is different from the pattern described in domestic cats. Lynx CLs produce progestagens in distinctive amounts permanently for at least two years, regardless of their origin (pregnancy or pseudo-pregnancy). It is suggested that long-lasting CLs induce a negative feedback to inactivate folliculogenesis, turning a normally polyestric cycle observed in most felids into a monoestric cycle in lynxes.     

Deer mouse aerobic performance across altitudes: effects of developmental history and temperature acclimation

Aerobic physiology at high altitudes has been studied in many animals. Prior work on laboratory-bred deer mice (a species with a wide altitudinal range) showed depression of aerobic capacity at high altitude, even after acclimation. However, wild deer mice show no reduction in thermogenic performance at high altitude, and performance limits seem to be due to physiological and anatomical adjustments to environmental temperature and not to oxygen availability. We asked whether across-altitude performance differences exist in deer mice after accounting for temperature acclimation (approximately 5 degrees and 20 degrees -25 degrees C) and prenatal and neonatal development altitude (340 vs. 3,800 m). We measured maximal thermogenic oxygen consumption (VO2sum) in cold exposure and ran mice on a treadmill to elicit maximal exercise oxygen consumption (VO2max). We found a 10% reduction in VO2max at 3,800 m compared with that at 340 m; thus, the mice were able to compensate for most of the 37% reduction in oxygen availability at the higher altitude. Development altitude did not affect VO2max. There was no effect of test altitude or development altitude on VO2sum in warm-acclimated animals, but both test and development altitude strongly affected VO2sum in cold-acclimated mice, and compensation for hypoxia at 3,800 m was considerably less than that for exercise.     

Effects of selenium toxicity on oestrous cyclicity, ovarian follicles, ovulation and foetal survival in rats

Effects of intraperitoneal injections of sodium selenite (2.0 and 4.0 mg/kg body weight) to normally cycling female albino Wistar rats daily for 30 days, and of single injection either during proestrous or oestrous and at each stage of the 4-day oestrous cycle were determined on oestrous cyclicity, ovarian follicles, ovulation, implantation and pregnancy outcome on day 14 of gestation. Administration of selenite for 30 days had no effect on the duration of first two oestrous cycles but afterwards the rats remained at the dioestrus stage. Their ovaries developed cystic follicles. Selenite treatments during the oestrous cycle preceding mating affects the implantation and pregnancy outcome in a dose-related manner. Its single dose containing 2.0 mg/kg body weight administered either at proestrous or oestrous, though had no effect on different reproductive parameters investigated in this study but its daily dose during the 4 day oestrous cycle reduced the number of corpora lutea and implantations as compared to saline injected control female rats. Similar effects of a single dose of selenite (4.0 mg/kg body weight) when injected at proestrous were recorded. Higher dose of selenite at oestrous or throughout the cycle decreased the number of implantations, but in addition, also increased the resorption rate/litter on day 14 of gestation. The present studies clearly show that high selenium levels in the body during the oestrous cycle preceding mating affects the number of ovulations, implantations and live embryos depending upon its dose and stage of administration.