Effects of neonatal androgenization on 7,12-dimethylbenz[a]anthracene (DMBA)-induced mammary tumorigenesis infemale noninbred Sprague-Dawley rats are reported. Testosterone propionate (1.25 mg) was injected sc into 29 2-day-old rats. At 50 days of age, all rats were given 20 mg of DMBA through a stomach tube. In these androgenized rats, no corpora lutea were found in the ovaries and the induction of mammary carcinoma by DMBA was strongly suppressed, whereas the induction of mammary dysplasia was significantly accelerated in comparison with the neonatally intact controls. Mammary dysplasia in the androgenized group varied widely, with two kinds of macroscopically detectable tumor-forming lesions (solid and cystic) and with microscopic characteristics of acinar adenosis, fibrotic adenosis, fibrosis, intraductal papillary proliferative lesions, and epithelial cysts. The earliest lesions of mammary dysplasia observed were acinar adenosis nodules and microcysts, both of which appeared as multifocal phenomena as early as 25 days after administration of DMBA.