The secretion rates of bioactive inhibin, oestradiol and progesterone were measured during the mid-luteal phase and at various times during the follicular phase of the cycle by a sensitive bioassay using sheep pituitary cells in culture in 12 Booroola ewes with and without copies of the Fecundity (F) gene in which the left ovary had been auto-transplanted to the neck.

Inhibin secretion was high during the luteal phase and fell in the early follicular phase in all genotypes (P < 0.01). In Booroola ewes with a F/- genotype, inhibin secretion then increased again, towards luteal rates, in the mid and late follicular phases. In Booroola ewes without a copy of the F gene (+/+) inhibin secretion remained low at all three sampling times in the follicular phase. The secretion rate of inhibin at 36 h (P < 0.1) and 48 h (P < 0.01) were significantly lower in ewes from the +/+ (no copy of the gene) ewes than in F/(one copy of the gene) ewes. Oestradiol secretion was low during the luteal phase and increased steadily during the early (24 h) to a plateau in the mid (36 h; P < 0.01) and late (48 h; P < 0.05) follicular phase. Progesterone secretion was high during the luteal phase, and decreased to a very low rate by 24 h after prostaglandin (PG) treatment (P < 0.001) and remained low. At 24 h after PG the concentration of FSH was significantly lower (P < 0.01) than that during the luteal phase and remained suppressed until the onset of the LH surge. There were no significant differences in LH concentrations. We conclude that (1) the secretion of inhibin by the ovary is highest in the luteal phase and (2) inhibin secretion is significantly raised during the mid to late follicular phase in Booroola ewes with a copy of the Fecundity gene compared with those without.