LRH, administered subcutaneously (s.c.) or orally (p.o.) to female rats, was capable of interfering with pregnancy when given on various days of gestation. A 100% inhibition of pregnancy was demonstrated at daily s.c. doses of 1000 mug/rat administered over days 1-7; LRH also was effective as an interceptive, terminating pregnancy in 100% of rats when delivered from days 7-12 of pregnancy at a daily dose of 1000 mug/rat s.c. or p.o. An 80% inhibition also was observed in rabbits administered LRH from days 1-7 at a total dose of 1000 mug/kg, s.c. Uterotrophic studies demonstrated that LRH, administered s.c. to intact immature mice, produces a dose-related increase in uterine and ovarian weight and initiates vaginal opening. The hypothalamic hormone also produced a dose-related increase in uterine weight in ovariectomized mice and hypophysectomized rats. The data suggest that LRH has a post-coital contraceptive effect, presumably acting via hyperstimulation of the hypophysial-ovarian steroid-uterine axis, and/or by a direct extrapituitary (uterine) effect.