Effect of photoperiod on LH, FSH and prolactin patterns in ovariectomized oestradiol-treated heifers

Angus and Angus crossbred heifers were ovariectomized, treated with oestradiol implants and randomly assigned to the natural photoperiod of fall to spring for 43 degrees N latitude or extra light simulating the photoperiod of spring to fall. Weekly blood samples were taken for 6 months (fall to spring equinox). All heifers were cannulated every 4 weeks and blood samples were taken for 4 h at 15-min intervals. Sera were assayed for LH, FSH, prolactin and oestradiol. In samples taken weekly, serum LH and FSH concentrations were higher while serum prolactin was lower in heifers exposed to natural photoperiod. There was a photoperiod X time interaction for both FSH and prolactin with concentrations diverging as photoperiod diverged. Circulating concentrations of oestradiol were not different between groups. In samples taken every 4 weeks at 15-min intervals, baseline concentrations of LH and FSH and LH pulse amplitude were higher while prolactin pulse frequency was lower in heifers exposed to natural photoperiod. There was a photoperiod X time interaction for each of these pulsatile characteristics. The correlation between LH and prolactin concentrations estimated from the 15-min samples differed between the two photoperiod treatment groups. The pooled correlation coefficient (r) was -0.12 under natural photoperiod and +0.50 under extra light. There was also a photoperiod X time interaction with negative correlations occurring when photoperiod was decreasing and positive correlations occurring when photoperiod was increasing. These results support the hypothesis that photoperiod alters serum concentrations of LH, FSH and prolactin in cattle.     

The effect of exogenous melatonin administration on gonadotropin and prolactin patterns in ovariectomized estradiol-treated heifers exposed to increasing photoperiod

Eight nulliparous Angus and Angus crossbred heifers, which had been ovariectomized and treated with estradiol-17beta (E(2)) S.Q. implants for 6 months, were used to determine the effects of exogenous melatonin on serum gonadotropin and prolactin concentrations. Melatonin (15 mg) or corn oil (vehicle) was administered as a single i.m. injection at 1600 h daily for 12 weeks (March 19 to June 4, 1982). Blood samples taken weekly via jugular venipuncture at approximately 1100 h were assayed for luteinizing hormone (LH), follicle stimulating hormone (FSH), prolactin and E(2). At 4-week intervals, animals were fitted with indwelling jugular cannulae at 1100 h and samples were taken for 4 h at 15-min intervals. These samples were used to estimate pulsatile patterns of LH, FSH and prolactin. On the day of the first 15-min sampling, additional blood samples were collected at 30 min intervals from 1500 to 2200 h to determine the acute effect of melatonin injection on concentrations of LH, FSH and prolactin. Melatonin did not affect concentrations of FSH collected at weekly intervals (P=0.03) but tended to inhibit the decrease in concentrations of LH seen in the heifers treated with vehicle (P=0.12). There was a melatonin x time interaction for FSH (P=0.04) and a tendency for this interaction for LH (P=0.11). Circulating concentrations of prolactin were not different between treatment groups (P=0.83) nor was there a melatonin x time interaction (P=0.03). Estradiol was higher in the melatonin treated group (P=0.03) (15.58 +/- 4.17 versus 8.25 +/- 1.25 pg/ml) (X +/- SEM) and the melatonin x time interaction was significant (P=0.001). There was a tendency for a melatonin x time interaction for FSH pulse frequency (P=0.10). Prolactin pulse duration tended to decrease in response to melatonin treatment (P=0.14) (15.92 +/- 9.29 versus 11.04 +/- 4.57 min). These data do not support the hypothesis that melatonin decreases prolactin concentrations in cattle and indicates that other factors must mediate photoperiod regulation of this hormone. However, the interpretation of these data is less clear concerning the hypothesis that melatonin may maintain elevated concentrations of gonadotropins in the presence of increasing photoperiod. Concentrations of FSH appeared to be more affected by melatonin than LH; consistent with previous observations that FSH may be more affected than LH by changes in photoperiod (2). But neither LH or FSH concentrations were clearly shown to be consistantly elevated in the melatonin treatment group.     

Effects of oestradiol on LH, FSH and prolactin in ovariectomized ewes

This study was conducted to test the hypothesis that the rate (dose/time) at which oestradiol-17 beta (oestradiol) is presented to the hypothalamo-pituitary axis influences secretion of LH, FSH and prolactin. A computer-controlled infusion system was used to produce linearly increasing serum concentrations of oestradiol in ovariectomized ewes over a period of 60 h. Serum samples were collected from ewes every 2 h from 8 h before to 92 h after start of infusion, and assayed for oestradiol, LH, FSH and prolactin. Rates of oestradiol increase were categorized into high (0.61-1.78 pg/h), medium (0.13-0.60 pg/h) and low (0.01-0.12 pg/h). Ewes receiving high rates of oestradiol (N = 11) responded with a surge of LH 12.7 +/- 2.0 h after oestradiol began to increase, whereas ewes receiving medium (N = 15) and low (N = 11) rates of oestradiol responded with a surge of LH at 19.4 +/- 1.7 and 30.9 +/- 2.0 h, respectively. None of the surges of LH was accompanied by a surge of FSH. Serum concentrations of FSH decreased and prolactin increased in ewes receiving high and medium rates of oestradiol, when compared to saline-infused ewes (N = 8; P less than 0.05). We conclude that rate of increase in serum concentrations of oestradiol controls the time of the surge of LH and secretion of prolactin and FSH in ovariectomized ewes. We also suggest that the mechanism by which oestradiol induces a surge of LH may be different from the mechanism by which oestradiol induces a surge of FSH.     

Effect of vascular cannulation on serum concentrations of LH, FSH, prolactin and cortisol in prepubertal gilts

Two experiments were conducted to determine whether cannulation of the jugular vein in gilts alters serum concentrations of LH, FSH, prolactin (PRL) or cortisol (C). In Experiment 1, 12 crossbred prepubertal gilts weighing 95 +/- 1.3 kg were immobilized by snaring, and tygon tubing was threaded into the anterior vena cava through a 12-gauge needle inserted into the jugular vein. Five hours later, blood samples were drawn at 20-min intervals for 4 h (Day 0). Samples were also drawn at 20-min intervals for 4-h periods 24 h (Day 1) and 48 h (Day 2) after cannulation. Serum concentrations of LH were similar (P=0.26) among Day 0 (0.40 ng/ml), Day 1 (0.39 ng/ml) and Day 2 (0.34 ng/ml). Serum PRL was similar (P=0.07) among Day 0 (4.10 ng/ml), Day 1 (3.87 ng/ml) and Day 2 (3.43 ng/ml). Serum concentrations of C were greater (P < 0.001) on Day 0 (8.32 ng/ml) than Day 1 (4.48 ng/ml) or Day 2 (3.54 ng/ml). In Experiment 2, cannulas were placed in 29 prepubertal gilts. Two days after initial cannulation, six blood samples were drawn at 20-min intervals. Gilts were then immobilized by snaring, and a second cannulae was inserted into the contralateral vein. Five blood samples were taken at 2-min intervals during the second cannulation and then six samples were drawn at 20-min intervals. Serum LH and FSH were not altered by cannulation or elevated during the subsequent 2-h sampling period (P>0.05). In contrast, serum concentrations of PRL rose slowly (P<0.05) during cannulation and remained elevated for 60 min before returning to baseline. Serum concentrations of C rose within 6 min of cannulation, remained elevated for 30 min, and then declined over the next 90 min. From these two experiments, it appears that secretory patterns of LH and FSH can be accurately assessed immediately after cannulation in prepubertal gilts. Measurements of serum PRL and C that reflect nonstressed conditions, however, cannot be obtained until at least 2 h or 1 d after cannulation, respectively.     

Effect of zearalenone and estradiol benzoate on serum concentrations of LH, FSH and prolactin in ovariectomized gilts

Six ovariectomized gilts were given zearalenone (Z), estradiol benzoate (EB) or vehicle in a replicated 3 x 3 Latin square design. Zearalenone was added to 2.3 kg of a corn-soybean ration at a dose of 1 mg Z/kg body weight; EB was given intramuscularly at 0.1 mg EB/kg body weight. Control gilts received vehicle solvent for both Z and EB. Blood samples were collected from indwelling jugular cannulas at 6-h intervals for 48 h before Z, EB or vehicle was given. After treatment, blood samples were drawn at 6-h intervals for an additional 84 h. Serum concentrations of luteinizing hormone (LH) decreased (P<0.001) from 4.67 ng/ml to 0.29 ng/ml within 6 h of EB. From 54 to 84 h after EB, serum concentrations of LH rose to 15.60 ng/ml (P<0.001). Serum concentrations of LH were reduced (P<0.001) in a similar pattern after Z (3.70 ng/ml to 0.49 ng/ml), but a rise in serum LH was not observed 54 to 84 h after Z (1.30 ng/ml). Serum concentrations of LH remained unchanged (P=0.55) in gilts given vehicle. Serum concentrations of follicle stimulating hormone (FSH) were suppressed (P<0.03) at 6 h in EB (19.10 vs 11.35 ng/ml) and Z gilts (16.16 vs 11.41 ng/ml) but remained unchanged in vehicle gilts. Serum concentrations of FSH did not change in EB or Z gilts during the next 36 h. These data indicate that the suppressive action of Z on serum concentrations of LH and FSH was similar to that of EB, while the biphasic stimulatory effect of EB for LH was not manifested by Z.     

Effect of progesterone on basal LH and episodic LH and FSH secretion in heifers

Heifers between Days 6 and 10 of the cycle were allocated at random to groups of 8 and treated with (i) a 4% progesterone-releasing intravaginal device (PRID) + oestrogen capsule for 12 days; (ii) 4% PRID for 12 days; (iii) 20% PRID for 12 days; (iv) 4% for 14 days; or (v) 20% PRID for 14 days. Blood was obtained daily during treatment and at 2- or 4-h intervals for 72 h after removal of PRIDs. Some animals were sampled every 20 min for 4.676 h on the 3rd day after PRID insertion, and 1 day before and 36 h after removal of the PRID insertion, and 1 day before and 36 h after removal of the PRID. During progesterone treatment there was: (i) no correlation between concentrations of progesterone and LH within days; (ii) a significant negative correlation between progesterone and days (P less than 0.01) and also between progesterone and LH over days (P less than 0.01); (iii) the overall correlation co-efficient between LH and days was positive (P less than 0.05). The amplitude of LH or FSH episodes was not affected as progesterone concentrations declined during PRID treatment, but the number of LH (but not FSH) episodes was increased (p less than 0.01). After PRID removal, the amplitude of both LH and FSH episodes increased (P less than 0.01). We suggest that progesterone is part of a negative feedback complex on LH secretion in cattle and that this effect is apparently mediated through frequency of episodic LH release.     

Effect of p-chlorophenylalanine on LH, FSH, prolactin and ovulation in the rat

The effect of p-chlorophenylalanine (PCPA: 300 mg/kg) on the rate of ovulation and plasma LH, FSH and prolactin secretion has been studied in rats at preovulatory periods (18th hour of diestrus) and post-ovulatory periods (9th hour of metaestrus). In both experimental groups, results showed that administration of PCPA caused an increase in both prolactin concentration and number of mature ovarian follicles (p less than 0.001). No changes were observed in FSH levels. LH concentration, however, decreased (p less than 0.001) and ovulation became totally inhibited. Rats treated at the 9th hour of metaestrus exhibited a marked luteinization as well as an increased number of corpus luteum in the ovaric tissue (p less than 0.001), whereas those treated at the 18th hour of diestrus underwent no luteinization and merely showed a greater number of mature ovarian follicles (p less than 0.001). PCPA, therefore, seems not to have a double effect on ovulation, LH, FSH, and prolactin secretion regardless of the pre or post-ovulatory periods. Changes observed in the ovaric tissue might be due to an increase in plasma prolactin concentration which appears earlier in the preovulatory than in the post-ovulatory treated animals. This difference may explain the double effect that has been attributed to the ovaric cycle and reproductive behavior.     

The circulating concentrations of FSH,LH and prolactin in the oestradiol-implanted ovariectomized ewe treated with caffeine

Caffeine, a trimethylxanthine alkaloid, is a psycho-active drug that effects a wide range of physiological systems, including the reproductive system. Reports of infants with intra-uterine growth retardation and lowered birth weight as a result of in utero exposure to caffeine, are increasing. The drug is also known to alter steroidogenesis but it is not certain whether this is a direct and/or indirect effect with the involvement of the central nervous system. Thus, an experiment was designed to determine the effect of acute caffeine administration on the circulating concentrations of gonadotrophins and prolactin in the ovariectomized oestradiol-implanted ewe. A single intravenous dose of caffeine (20 mg kg⁻¹ bodyweight) did not affect circulating gonadotrophin concentrations with the parameters for the pulsatile secretion of luteinizing hormone (LH) and the mean concentration of follicle stimulating hormone (FSH) being similar in both experimental and control groups. Circulating prolactin levels, on the other hand, were significantly (P < 0.01) elevated following intravenous treatment with caffeine. The effect was immediate following caffeine administration with elevated concentrations being maintained over the next 3 h before their return to pre-treatment concentrations. The response was bi-phasic with peaks of prolactin concentrations at 1 and 3 h. The results of this experiment show that acute caffeine exposure does not affect the secretion of gonadotrophins from the anterior pituitary gland. Furthermore, they show that acute administration of caffeine stimulates prolactin secretion via an action that is independent of oestradiol feedback and which we suggest, may involve the ACTH/adrenal axis.     

Steroid hormone feedback on LH in prepubertal Holstein heifers

A significant dose-response relationship between gonadotropin-releasing hormone (GnRH) and time to luteinizing hormone (LH) peak, peak serum LH and total serum LH was obtained in prepubertal Holstein heifers (28 weeks of age) (Experiment 1). For the second experiment, the effect of steroid feedback on the anterior pituitary was determined. A steady infusion of saline, estradiol-17 beta or progesterone was maintained for 24 h while GnRH, in various schemes, was administered 8 h after the beginning of steroid infusion. Estradiol-17 beta infusion (2.08 micrograms/h), although it did not affect peripheral concentrations of estrogen, caused an LH release 24 to 30 h later in 37.5% of the heifers. This amount of exogenous estrogen did not affect the LH response to a single GnRH (4 micrograms) challenge. When the same GnRH dosage (4 micrograms) was administered 6 times at hourly intervals, the heifers infused with estradiol had a lower response after the first 2 injections of GnRH and a greater response after the last 4 injections than heifers infused with saline. When GnRH was infused (4 micrograms/h) for 6 h, beginning 8 h after steroid infusion, estradiol infusion caused a significantly higher peak LH and total LH release than an infusion of either saline or progesterone (7.3 micrograms/h). The progesterone infusion had no effect on the GnRH-stimulated LH release. We conclude that prepubertal dairy heifers have an anterior pituitary capable of responding to the feedback effect of estrogen in a positive manner.     

Effect of ethanol in preovulatory periods on LH, FSH, prolactin and ovulation in rats

The effect of ethanol (4 g/kg) as well as the role of serotoninergic neurons on the rate of ovulation and plasma LH, FSH and prolactin secretion have been studied in rats at preovulatory periods (18th hour of diestrus). It has been found that administration of ethanol in preovulatory periods decreased the number of ovules per rat (p less than 0.001), the number of ovulating rats and LH levels (p less than 0.001). These effects were accompanied by an increase in prolactin concentration (0.05 greater than p greater than 0.02), which was followed by a diffuse luteinization in the ovarian tissue. These results showed that ethanol had an effect of central depression in preovulatory periods. These effects could be mediated through the hypothalamic releasing factors. Under previous serotonin depletion with p-chlorophenylalanine (PCPA: 300 mg/kg), ethanol caused similar effects on LH and FSH levels as compared with the control group with PCPA. However, prolactin concentration was not increased. These results showed that serotoninergic neurons could be mediated in changes caused by ethanol on prolactin secretion, but do not affect directly in changes caused on LH and FSH secretion.     

Plasma FSH and LH in prepubertal Booroola ewe lambs

Basal plasma concentrations (four 30-min samples) and GnRH-induced release of gonadotrophins were measured every 15 days between 30 and 90 days and at 110 days of age in Merino ewe lambs from the prolific Booroola ('B') flock (n = 18-23), the medium prolificacy ('T') flock (n = 14-20), and the 'O' flock (n = 4-8) of low prolificacy. At ages of 30 and 45 days B ewe lambs had mean basal plasma FSH concentrations of 145 and 122 ng/ml which were significantly higher (P less than 0.01) than those seen in T (45 and 53 ng/ml), and O (39 and 38 ng/ml) flock ewes. Between 60 and 110 days of age there were no significant differences between genotypes. The increment in FSH concentrations above basal levels induced by the subcutaneous injection of 100 micrograms synthetic GnRH was only significantly (P less than 0.05) greater in B than T and O genotype ewe lambs at 110 days of age but not at other ages. The basal plasma FSH differences between the B, T and O genotypes at 30 and 45 days of age were not consistently related to the size of litter in which lambs were born. At 30 days of age the mean plasma LH concentration of B, T, and O flock lambs were 2.6 +/- 0.5, 1.2 +/- 0.6 and 0.7 +/- 0.8 ng/ml respectively. These differences were not significant. At later ages there were also no significant differences between the genotypes with respect to basal LH, and the increase in LH induced by exogenous GnRH was always similar for the three genotypes.(ABSTRACT TRUNCATED AT 250 WORDS)     

Opioidergic, dopaminergic and adrenergic regulation of LH secretion in prepubertal heifers

Studies have shown inhibitory effects of endogenous opioids on LH secretion in early post-natal heifers. However, it is not clear whether these effects change during the rest of the prepubertal period or whether the inhibitory influences on the GnRH neurones are direct or by way of other neuronal systems. Two experiments were performed in heifer calves to study the developmental patterns of opioidergic, dopaminergic and adrenergic regulation of LH and the possible interactions between opioids and dopaminergic and adrenergic neuronal systems, in the regulation of LH secretion. In Expt 1 four groups each of five heifer calves were used. Blood samples were taken every 15 min for 10 h and each calf received one of the following treatments as a single injection at 4, 14, 24, 36 and 48 weeks of age: (i) naloxone (opioid antagonist, 1 mg kg(-1), i. v.); (ii) sulpiride (dopamine D2 antagonist, 0.59 mg kg(-1), s.c.); (iii) naloxone and sulpiride combined; or (iv) vehicle (control group). Treatments began after the first blood sample was taken. The design of Expt 2 was similar; a separate group of heifer calves was assigned to receive one of the following treatments as a single injection at 4, 14, 24, 36 and 48 weeks of age: (i) naloxone; (ii) phenoxybenzamine (an alpha-adrenoreceptor blocker, 0.8 mg kg(-1), i. v.); (iii) naloxone and phenoxybenzamine; (iv) or vehicle. Results from Expt 1 showed that the maximum concentration of LH and the number of calves responding to treatments with an LH pulse was higher in the first hour after treatments at 36 and 48 weeks of age in the naloxone group compared with the control or sulpiride groups (P < 0.05). These values in the naloxone group also increased over time and were greatest at 48 weeks of age (P < 0.05). In heifers given naloxone + sulpiride treatment at 36 and 48 weeks of age, maximum concentrations of LH in the first hour after treatment did not differ from the naloxone and control groups. In Expt 2, at 36 and 48 weeks of age, treatment with naloxone with or without phenoxybenzamine resulted in higher concentrations of LH than in the controls (P < 0.05). No pulses were seen over the first hour of treatment at 36 and 48 weeks of age in heifers treated with phenoxybenzamine. The 10 h periods of blood sampling at 48 weeks of age revealed that phenoxybenzamine alone suppressed LH pulse frequency and mean serum concentrations of LH compared with the control group (P < 0.05). It was concluded that a strong or more acute inhibition of LH secretion by endogenous opioids developed in mid- to late prepubertal heifers, or alternatively, that removal of opioidergic inhibition at the GnRH neurone unmasked stimulatory inputs that were greater in heifers close to first ovulation. Since sulpiride appeared to negate in part the effects of naloxone on LH release, the suppressive effects of opioids could be exerted in part through the inhibition or blocking of a stimulatory dopaminergic system. alpha-Adrenergic neuronal systems have stimulatory effects on LH release, especially during the late prepubertal period, but do not appear to mediate opioidergic inhibition of LH secretion in prepubertal heifer calves.     

Effect of sex on annual variations in plasma LH and FSH levels in prepubertal subjects

We studied, from 1977 to 1979, 61 females and 72 males (aged 6 to 10 years) in order to demonstrate the occurrence of FSH and LH circannual variations. The data were fitted a cosine function by least square method in order to describe any rhythm and to estimate its parameters:mesor, amplitude, acrophase. Our data suggest that in prepubertal age the behaviour of FSH secretion is different in two sexes, but without circannual rhythm. LH instead shows a statistically significant circannual rhythm in both groups, without differences in mean levels between the two sexes.     

Post partum patterns of circulating FSH, LH, prolactin, estradiol, and progesterone in nonsuckling cynomolgus monkeys.

Circulating levels of FSH, LH, prolactin (Prl), estradiol (E), and progesterone (P) were determined by RIA in four intact and four monkeys luteectomized (CLX) at parturition in order to a) characterize the patterns of these hormones during the puerperium, and b) examine a possible inhibitory role of the "rejuvenated" corpus luteum (CL) on the resumption of follicle growth post partum. In both groups during the first four weeks, FSH and LH were at tonic levels typical of ovulatory cycles. Recurrent puerperal "surges" of FSH, but not LH, unaccompanied by increments in serum E, were observed in both intact and CLX monkeys. No consistent pattern of serum Prl was apparent. CLX was followed by a prompt fall in serum P levels, which were elevated above typical follicular phase levels into the second week post partum in intact monkeys. Menstrual cycles resumed 2-4 months after delivery. Hormonal patterns during the first menstrual cycle post partum were indistinguishable from those observed in pregravidic ovulatory cycles. The findings indicate that in nonsuckling cynomolgus monkeys a) although it secretes progesterone, the puerperal CL does not inhibit the resumption of the ovarian cycle post partum, b) the puerperal ovary is not absolutely refractory to gonadotropins, since initial trials with Pergonal + hCG stimulated ovarian function, and c) ovarian activity during the puerperium may be limited by factors other than the tonic supply of gonadotropins.     

Plasma LH, FSH, TSH, prolactin and cortisol during a combined test of pituitary reserve in prepubertal subjects with short stature]

The effect of a combined test (insulin, TRH, LHRH) on plasma concentrations of LH, FSH, PRL, Cortisol was studied in 18 normal subjects in prepubescent state and in 65 patients in prepubescent state with short stature. In 6 subjects with short stature there was no change of LH, and in 7 subjects with short stature there was no change of FSH. In conclusion the combined test for pituitary stimulation provides an useful method for localizing the lesion in disorders of hypothalamo-pituitary axis.     

Effect of prolactin on the LH response to synthetic LH-RH in ovariectomized ewes.

When ovariectomized ewes were treated with LH-RH, all 3 receiving prolactin infusion and 4 out of 5 receiving an infusion of NaCl solution responded.     

Roles of the dominant follicle and the pattern of oestradiol in induction of preovulatory surges of LH and FSH in prepubertal heifers by pulsatile low doses of LH

Prepubertal crossbred beef heifers were injected (i.v.) with 50 micrograms bovine LH every 2 h for 48 h (first injection at 0 h). At 28 h, number and diameter of ovarian follicles were determined by ultrasonic scanning, and unilateral removal of either the ovary bearing the largest follicle (Group UL, N = 5) or the opposite ovary (Group UO, N = 4) was performed; control animals remained intact (Group I, N = 5). Blood samples were taken every 2 h (starting at 0 h) for a 60-h period to assess concentrations of gonadotrophins and oestradiol. Preovulatory-like surges of LH occurred in 0/5, 4/4 and 5/5 heifers for Groups UL, UO and I respectively; the time of the LH surge did not differ between animals in Groups I and UO (mean = 40 h). FSH in Group UL heifers rose to a plateau immediately after unilateral ovariectomy; this pattern was not observed in the other two groups (P less than 0.01). The area under the curve for FSH was significantly different (P less than 0.05) among groups after 28 h. Preovulatory-like surges of FSH occurred coincidently with those of LH, except for one Group I heifer. An increase in the concentrations of oestradiol between 0 and 28 h was detected in all animals. Profiles of oestradiol during this period did not differ between heifers that had an LH surge (Group UO and I) and those that did not (Group UL).(ABSTRACT TRUNCATED AT 250 WORDS)     

Effect of beta-adrenergic drugs on LH, FSH, and growth hormone (GH) secretion in conscious, ovariectomized rats

The effects of third ventricular (3V) injection of the beta-adrenergic antagonist, propranolol (PROPR), a selective beta 1-antagonist, metoprolol (MET), a selective beta 2-antagonist, IPS 339, and a beta-adrenergic agonist (-) isoproterenol (ISOPR), on plasma concentrations of luteinizing hormone (LH), follicle stimulating hormone (FSH), and growth hormone (GH) were studied in conscious, ovariectomized (OVX) rats. Samples were removed from unrestrained rats which had been previously implanted with atrial and 3V cannulae, and plasma hormone levels were determined by radioimmunoassay (RIA). Intraventricular injection of PROPR (30 micrograms), MET (40 micrograms), or IPS 339 (20 micrograms) induced a gradual elevation in plasma GH concentrations, whereas ISOPR (30 micrograms) reduced plasma GH. ISOPR (30 micrograms) brought about a decrease in plasma LH concentrations, but PROPR, MET and IPS 339 had no effect on LH levels. PROPR (30 micrograms) increased plasma FSH concentrations, but there was no significant effect of MET, IPS 339 or ISOPR on FSH secretion. The results indicate that the beta-adrenergic system can inhibit the release of GH, LH, and FSH. This system appears to have a tonic inhibitory effect on GH and FSH but not LH release in the OVX rat.