Administration of steroid-free bovine follicular fluid (bFF), containing inhibin-like activity, depressed levels of FSH measured 4 h after injection in intact adult and 35-day-old female rats, but not in younger females. Suppression of FSH was also observed in intact male rats, aged 55 days, but not in older and younger male rats. Eight hours after injection of bFF, FSH levels were depressed in 15-day-old and older immature and adult rats of both sexes. Male and female rats, gonadectomized 2 days earlier, responded similarly to bFF treatment as did the intact animals. In a second experiment it was found that the rise of FSH levels, occurring within 8 h of gonadectomy, decreased with age in male and increased with age in female rats. Steroid treatment was found to prevent the rise in FSH levels partially in 15-day-old male and completely 25-day-old female rats, whereas treatment with bFF was fully effective in blocking the FSH rise in both immature and adult rats of both sexes. It is concluded that inhibin might be a major physiological factor in a fast-acting control of FSH concentrations from at least the age of 25 days onwards in female rats. In male rats its physiological significance might be limited to the prepubertal period, despite the fact that pituitary secretion of FSH is suppressed by exogenous inhibin-like activity at all ages studied.

The time-related changes in gonadotrophin concentrations after a single injection of steroid-free bovine follicular fluid (bFF), which contains material with inhibin-like activity, were studied in 25-day-old and adult female rats which either were intact or had been ovariectomized 2 days before. In ovariectomized and intact rats administration of bFF caused a selective suppression of FSH after 4--8 h in 25-day-old rats and after 3--4 h in adult rats. No systematic changes in concentrations of LH after bFF injection were observed. Relative suppression of FSH levels in adult rats was more pronounced and of longer duration than in 25-day-old rats. Moreover, the total period of suppression lasted longer in ovariectomized than in intact rats (12 and 8 h for 25-day-old and 24 and 15 h for adult rats respectively). Hypersecretion of FSH was found in intact rats after the initial suppression; this phenomenon was more pronounced and of long duration in adult than in 25-day-old rats. No clear change in the numbers of healthy growing follicles was observed after injection of bFF into intact rats. These results indicate that the pituitary secretion of FSH responds quickly and selectively after administration of bFF to intact and ovariectomized, 25-day-old and adult female rats. The hypersecretion of FSH in intact rats might compensate for the initial suppression of this gonadotrophin, and may thus ensure the maturation of a normal number of follicles.

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SUMMARYTo determine whether the high FSH levels present before day 20 in female rats are of importance for normal follicular development, the effect of depression of FSH levels on numbers of medium and large follicles was studied. Two Injection of antiserum to FSH from day 7 till day i i resulted in a decreased number of large follicles on day 12 . The same treatment given from day i till day 15 or from day r 5 till day 19 did not induce this effect, although one injection on day I or day r 5 induced atresia in many large follicles one day later. This acute effect seems to be compensated within five days.The present results support the view that the high gonadotrophin levels (especially FSH) before day 20 are a requirement for normal follicular development.