Acute suppression of FSH secretion by oestradiol in the ovariectomized rhesus monkey

Using long-term ovariectomized rhesus monkeys, we examined the ability of oestradiol to decrease circulating FSH concentrations in the absence of other ovarian factors. Daily blood samples were obtained from untreated monkeys for 8 days, followed by insertion of oestradiol capsules after the Day-8 sample was taken. Samples were then taken on Days 9-15, the capsules were removed after the Day-15 sample, and samples were obtained on Days 16-19. Serum was assayed for concentration of oestradiol, FSH and LH by RIA. The concentration of FSH (ng/ml) in serum did not change during the first 8 days before oestradiol treatment (overall mean = 356 +/- 55) but decreased from the Day-8 value of 320 +/- 8 to 190 +/- 42 on Day 9 and by Day 15, after 7 days of oestradiol treatment, had reached a nadir of 20 +/- 5. By Day 17, i.e. 2 days after removal of the oestradiol capsules, serum FSH had increased (P less than 0.05) to 92 +/- 23 with a further increase (P less than 0.05) on Day 19 (171 +/- 16). This study demonstrates that, unlike in rats, mice, and sheep, administration of oestradiol alone to ovariectomized rhesus monkeys reduces immunoreactive serum FSH to concentrations measured in intact animals.