Postcoital antifertility efficacy, estrogenic and antiestrogenic activities of compound 85/287 were determined by the subcutaneous route in rats. It was 100% effective in preventing implantation at 0.5 mg/kg dose when administered within 24 h of mating and at 0.05 mg/kg in the days 1-5 post-coitum regimen. In the immature rat bioassay, it exhibited mild uterotrophic effect at the contraceptive dose but when administered along with estradiol (E2), it caused almost complete inhibition of uterine weight gain and vaginal cornification at the 2 mg/kg dose. E2 administration to immature rats (0.1 microgram, s.c., 3 days) caused 3-5 fold increase in the nuclear as well as cytoplasmic estradiol receptor (ER) content as compared to controls. In contrast, 85/287 (0.5 mg/kg and 2 mg/kg; s.c.), only translocated the ER to the nuclear compartment resulting in a depletion of cytoplasmic ER levels. Concurrent administration of 85/287 and E2 inhibited E2-induced increase in cytoplasmic ER. It is suggested that compound 85/287 exerts its antiestrogenic and antiimplantation action by interfering with the formation of E2-receptor complexes in the uterus.