In late-prepubertal female rats passive immunoneutralization of endogenous inhibin was achieved by injection of inhibin antiserum. Effects of follicle population, timing of sexual maturation, ovulation rate at first and second oestrus and serum FSH levels were studied. Rats were injected with antiserum, (non-immune) control serum from castrated sheep (castrated serum) or their IgG fractions, or with saline on day 33 or 3 or 2 days (days -3/-2) before the expected day of first ovulation, day 38.5 ± 0.2 (n=70). Blood was collected from different subgroups at 8, 24 and 48 h, and at first and second oestrus after injection. At necropsy, ovaries were histologically prepared for differential counting of follicles (48 h and first oestrus) and counting of corpora lutea (CL; first and second oestrus) as an index of ovulation rate. Results from rats injected with either serum or its IgG fraction were not different, as was the case when rats were injected with either castrated serum or saline. Thus, results from groups treated with antiserum and antiserum IgG were combined and labelled 'antiserum', and the castrated serum, castrated serum IgG and saline-treated groups were combined and labelled 'control'. The activity of inhibin-neutralizing antibodies in the circulation of antiserum-treated rats was reduced by 43% between 8 h and second oestrus after injection, as determined by the binding of purified bioactive radioiodinated 31 kDa bovine inhibin. After antiserum injection on day 33, more healthy antral follicles (vol. >100 x 105 μm3, diameter >260 μm) were present in the ovaries at 48 h (70.6 vs 54.5; P < 0.05) and at first oestrus (73.1 vs 50.8; P < 0.05) if first oestrus was reached within 5 days, but numbers were not different if first oestrus was more than 5 days after injection (52.6 vs 50.8). The number of CL after injection of antiserum on day 33 was increased at first oestrus compared with control (13.4 ± 0.5, n = 30, vs 10.0 ± 0.2, n = 40; P < 0.001), an effect that was even more clearly present in antiserum-injected rats ovulating within 5 days (14.4 ± 0.7, n = 20; P < 0.001). Rats injected with antiserum at days -3/-2 showed a doubling of ovulation rate at first oestrus when compared with control animals (21.5 ± 0.8, n = 12, vs 10.5 ± 0.2, n = 15; P < 0.001). No differences in the number of CL was seen at second oestrus. Age and body weight on the day of first ovulation were not influenced by antiserum treatment. Serum FSH was significantly (P < 0.01) increased at 8 h after antiserum injection on either day 33 or on days -3/-2 to a level of 250 and 800% of control levels respectively. Thus, injection with inhibin-neutralizing antiserum into prepubertal female rats resulted, through an increase in serum FSH concentration 8 h after injection, in the growth of additional numbers of healthy antral follicles. Supranormal ovulation rate occurred if antiserum injections were given within the last 5 days before first ovulation, with a maximal ovulation rate after injection on days -3/-2. The data support the view that, in the immature female rat during the last 5 days before the day of first ovulation, inhibin is (through its regulation of serum FSH levels) progressively involved in the control of follicle growth and ovulation rate.