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Passive immunoneutralization of LHRH by injecting a caprine antiserum to LHRH (LHRH-AS) in lactating rats nursing two pups on day 5 of lactation resulted in an immediate decline in concentrations of LH in the plasma during the 24-h study period, followed by a gradual increase to control levels 30 h later. Concentrations of oestradiol-17β and inhibin activity in ovarian venous plasma also decreased abruptly in LHRH-AS treated animals and recovered to control levels 36 h later. These changes were correlated with changes in concentrations of LH in the plasma. On the other hand, plasma concentrations of FSH increased abruptly in the LHRH-AS treated animals within 3 h after injection, but the concentrations declined gradually to control levels 48 h later. The ability of follicles to ovulate in response to human chorionic gonadotrophin (hCG) began to decrease within 6 h after treatment with LHRH-AS, and further decreased until 18 h after injection of LHRH-AS, when hCG induced ovulation (with two oocytes) in only one of five animals. A gradual increase in ovulation rate to control levels was noted by 36 h after injection of LHRH-AS. These results indicate that Graafian follicles present at the time of LHRH-AS injection had become atretic and that a new set of follicles had then begun to mature. Selective release of FSH could not be induced by injection of LHRH-AS in ovariectomized animals. Treatment with inhibin (porcine follicular fluid) suppressed the selective release of FSH, whereas treatment with oestradiol-17β had no inhibitory effect on the selective release of FSH.

These findings indicate (1) that tonic secretion of LH is an important factor in normal follicular maturation and maintenance in lactating rats, (2) that selective release of FSH after injection of LHRH-AS is attributed to the removal of a negative influence of inhibin from antral follicles during the period of follicular atresia and (3) that a selective surge of FSH is responsible for initiation of new follicular maturation.

J. Endocr. (1988) 118, 455–464