Summary. Experiments were conducted with ewes to investigate the effects of an enriched bovine follicular fluid inhibin preparation (INH) on gonadotrophin secretion after the onset of oestrus. Administration of INH (10 mg) 1 h after the onset of oestrus did not significantly alter the preovulatory FSH and LH surges or the second FSH peak. To determine the effects of INH on the second FSH surge, ewes were treated with saline (N = 7) or INH (N = 10) at 4 h (10 mg) and 24 h (5 mg) after the peak of the preovulatory LH surge. The second FSH surge was delayed about 24 h (P < 0·05) in ewes treated with INH; however, the delay did not alter the interval to the next oestrus.

In a third experiment, 16 ewes were assigned to 4 groups in a 2 × 2 factorial with the main effects being ovariectomy at 4 h and INH treatment (10 mg) at 4, 20 and 36 h after the peak of the LH surge. Controls received sham ovariectomy and saline injection as appropriate. Ovariectomy resulted in a rapid increase in serum FSH but not LH and this was delayed (P < 0·05) by INH treatment. These results indicate that inhibin has a selective inhibitory action on FSH secretion in ewes and suggests that the second FSH surge results from increased basal FSH secretion due to decreased endogenous inhibin levels.

The effects of charcoal-extracted bovine follicular fluid (BFF) on endocrine profiles and follicular development in intact and hemiovariectomized postpubertal heifers were examined. Oestrus-synchronized heifers received Norgestomet implants on day 1 and 7 of treatment and were then injected s.c. with 11 ml saline (control) or 11 ml BFF twice a day for 12 days. The ovary bearing the largest follicle (OV₁) was removed on day 7 and the remaining ovary (OV₂) was collected on day 13. Follicles were observed by daily ultrasonography and were classified according to diameter (size 1: 2–3 mm; size 2: 4–6 mm; size 3: 7–10 mm; size 4: > 10 mm). After ovariectomy they were classified by diameter and histologically as normal or atretic. Intact control heifers had increased numbers of size 4 follicles on OV₁ on days 6 and 7; no increase was observed in BFF-treated heifers (P < 0.03). In BFF-treated heifers, the mean basal LH concentration was higher (P < 0.05) and that of FSH was lower (P < 0.04) than in controls. FSH concentrations in BFF-treated heifers decreased from 0.60 ± 0.08 ng ml⁻¹ (day 1) to 0.22 ± 0.05 ng ml⁻¹ (day 7; P < 0.04). The concentration of oestradiol increased in control heifers, but not in BFF-treated heifers (P < 0.001). After hemicastration, OV₂ underwent compensatory hypertrophy in control heifers, with an increase in the number of size 2, 3 and 4 follicles (P < 0.05), whereas BFF-treated heifers did not. Thus, total follicular volume was much lower in BFF-treated than in control heifers on day 13 (92.2 ± 15.4 versus 1393.8 ± 276.6 mm³; P < 0.0002). A transient increase in FSH (P < 0.006) and oestradiol (P < 0.01) concentrations occurred after hemiovariectomy in control but not in BFF-treated animals. In control heifers, an analysis of temporal relationships showed negative correlations between the volume of size 3 and size 4 follicles, and between FSH concentrations and the volume of size 3 and 4 follicles. A positive correlation was found between the mean diameter of the largest follicle and the concentration of oestradiol, whereas negative relationships were found between the concentrations of FSH and oestradiol, and between FSH and the mean diameter of the largest follicle. Analysis of the histological data showed that the number and volume of follicles > 8.57 mm was lower in the BFF-treated OV₁ ovary, whereas no differences were found for follicles ≤ 8.57 mm. An absence of or a lower number and volume of follicles ≥ 3.68 mm was observed in BFF-treated OV₂ compared with controls. We showed that: (i) compensatory hypertrophy in the remaining ovary occurs in heifers when the ovary bearing the largest follicle is removed; (ii) treatment with BFF decreases the FSH concentration and follicular development in intact postpubertal heifers, and prevents compensatory FSH increase and follicular growth after unilateral ovariectomy; and (iii) the histological population of follicles < 3.68 mm in diameter are not altered following 12 days of BFF treatment. Our results suggest that follicular development beyond 3–4 mm in cattle depends on an adequate circulating concentration of FSH.