Female Sprague-Dawley CD rats were injected s.c. with 5 mg genistein, a soy phytoestrogen, or 20 μl of the vehicle, dimethylsulfoxide (DMSO), on days 2, 4 and 6 postpartum. At day 50, they were exposed to 80 μg dimethyl-benz[a]anthracene (DMBA)/g body wt. Animals treated neonatally with genistein as compared to DMSO had increased latency and reduced incidence and multiplicity of DMBA-induced mammary adenocarcinomas. Mammary whole mount analysis showed that 50 day old female rats treated neonatally with genistein had fewer terminal end buds. Cell proliferation studies revealed that 50 day old genistein-treated rats had lower percentages and total numbers of cells in the S-phase of the cell cycle in terminal end buds, terminal ducts, lobules I and lobules II. In genistein-treated as compared to vehicle-treated female rats, vaginal openings occurred earlier, the estrus cycle was disrupted and the uterine-ovarian weights were smaller. In 50 day old genistein-treated females there were atretic antral follicles, fewer corpora lutea, and lower circulating progesterone but not estradiol-17β concentrations. In 21 day old rats treated neonatally with genistein, mammary glands were larger and there were more terminal end buds and terminal ducts, and more proliferative activity in all terminal ductal structures. It appears that neonatal genistein-treatment exerted its chemoprevention action by acting directly to enhance maturation of terminal ductal structures and by altering the endocrine system to reduce cell proliferation in the mammary gland. © 1995 Oxford University Press.