Immature, 32-day-old rats were treated with PMSG. Stress consisting of 180 min immobilization administered during the period of the preovulatory LH surge resulted in the blockage of ovulation. The anti-ovulatory action of stress was effectively antagonized by the administration of naloxone hydrochloride shortly before the application of the stress, suggesting that the ovulation block by stress is mediated by endogenous opioids known to be elevated during stress. Blockage of ovulation by 180 min immobilization stress occurred only in rats from one of the two laboratory strains tested, suggesting that stress resistance is, at least in part, genetically determined.