The estrogen-like activity of delta-9-tetrahydrocannabinol (delta9-THC), an active component of marihuana, as measured by uterine weight gain and vaginal smear techniques in ovariectomized rats, is reflected in histological examination of uterine and vaginal tissues. Doses of 1, 2.5, and 10 milligrams of delta9-THC per kilogram elicit hypertrophy and hyperplasia of the uterus; the dose of 2.5 milligrams per kilogram is most effective. There is an increase in stratification of vaginal epithelium with doses of 2.5 and 10 milligrams per kilogram; cornifying cells are seen with 2.5 milligrams per kilogram.