Anordrin, administered in a single s.c. dose of 62.5 micrograms in sesame oil, stimulated sustained uterine growth (wet weight) when measured at 24 and 72 hr, but total soluble protein and total DNA per uterus was not increased. By comparison, 3 micrograms of estradiol-17 beta under the same conditions significantly increased all three parameters of uterine growth. Both of the above steroid treatments significantly increased nuclear estrogen receptor content of the uterus, but only the estradiol-17 beta treatment resulted in significantly elevated cytosol receptor content per uterus. Anordrin binds to the 8S estrogen receptor with an affinity of about 2 x 10(5) M-1 as determined by competition with [3H]estradiol-17 beta. The abortifacient activity of Anordrin when given orally (8 mg/kg b.w.) to mice on the 7th day of pregnancy was almost completely blocked by simultaneous oral administration of estradiol-17 beta (0.8 mg/kg b.w.). It is concluded that the actions of Anordrin on the uterus can be attributed to its antiestrogenic activities.