Female 2-day-old Sprague-Dawley rats were given a subcutaneous administration of either 1.25mg of testosterone propionate, 1.0mg of 5α-dihydrotestosterone, 0.1mg of 17β-estradiol, or 1.0mg of progesterone. At 50 days of age, all the rats were given 20mg of 7, 12-dimethylbenz[a]anthracene (DMBA) by a stomach tube. All the rats were killed at 300 days of age. In the rats given testosterone propionate or 17β-estradiol, lactation in the mammary gland and absence of corpora lutea in the ovaries were found. Induction of mammary carcinoma by DMBA was strongly suppressed, whereas the induction of mammary dysplasia was significantly accelerated in the rats given either testosterone propionate or 17β-estradiol. Both 5α-dihydrotestosterone and progesterone showed no remarkable changes in the induction of mammary carcinoma or dysplasia.