The long-term effects on mammary glands of 5beta-dihydrotestosterone, considered to be biologically inactive, were studied in female SHN mice with mammary tumor, virus. 5beta-Dihydrotestosterone was adminstered to mothers for 4 days from Day 12 to Day 15 of pregnancy (prenatal treatment) and to pups for 5 days of postnatal life (neonatal treatment) at daily doses of 1 mg and 200 mug, respectively. Neonatal treatment resulted in marked stimulation of spontaneous mammary tumorigenesis; all neonatally treated mice had palpable mammary tumors by 6.2 months of age, when mammary tumor incidences in the control mice and in mice treated prenatally with 5beta-dihydrotestosterone were 21.1 and 6.3 percent, respectively. Furthermore, neonatal treatment promoted normal and preneoplastic mammary growth and pituitary prolactin secretion and induced ovarian anovulatory syndrome in all mice. Prenatal treatment also increased the number of mammary hyperplastic alveolar nodules and induced an delayed anovulatory syndrome. These results have demonstrated that perinatal treatment with 5beta-dihydrotestosterone can induce such irreversible changes in the mammary glands, pituitary gland, ovaries, and genital tracts as those seen with other biologically active steroid hormones.