Progesterone stimulated the growth of mammary carcinomas previously induced by 7,12-dimethylbenz[a]anthracene (DMBA) in neonatally androgenized female Sprague-Dawley rats. This stimulatory effect appeared to depend on the action of estrogens. At 2 days of age, 199 female rats were neonatally androgenized by single sc injections of 1.25 mg testosterone propionate. At 50 days of age, all rats were given 20-mg doses of DMBA by gastric intubation. About one-half of the androgenized rats were ovariectomized at 78 days of age. Rats in test groups were given daily im injections of 10 micrograms 17 beta-estradiol or 4 mg progesterone for 228 days starting at 78 days after birth. The experiments were terminated when the rats were 300 days of age. In all androgenized rats, corpora lutea were absent. In the endrogenized rats, progesterone injection induced a significant and marked increase in the incidence of mammary carcinomas from that in the androgenized controls (3.4%) in the presence of ovaries (90%) but not in the absence of ovaries (8.7%). Estrogen injecton showed some but no significant increases in the incidence of mammary carcinoma in both ovariectomized and nonovariectomized androgenized rats (17.4 and 40.0%, respectively).