A study was made of the effects on the preovulatory surges of LH and FSH exerted by morphine sulfate (M) administered at 1400 h on proestrus, the beginning of the critical period for gonadotropin release in cycling rats in our colony. Various doses of M were given ip and jugular venous blood was collected under ether anesthesia for radioimmunoassay of plasma levels of LH and FSH. To verify ovulation, oviducts were examined the following day for the presence of ova. A high dose of M (60 mg/kg) totally abolished the LH surge and partially prevented the FSH surge while completely blocking ovulation. With intermediate doses of M (20-30 mg/kg) about half of the rats showed undiminished elevations of plasma LH at 1800 h and complete ovulation the next morning, whereas the other half revealed almost total blockade of LH release and no ovulation. Following a low dose of M (10 mg/kg) a small but significant (P