The effects of suramin on the growth of seminal vesicles (SVs) of neonatal mice were investigated in vitro. SVs from 0-day-old male mice were cultured in serum-free chemically defined medium supplemented with 5 alpha-dihydrotestosterone (DHT, 10(-8) M) and insulin (10 micrograms/ml), alone and in combination. Prior to culture, SVs from 0-day-old mice had no epithelial branches. SVs cultured in medium with DHT formed numerous epithelial branches, while epithelial branching did not occur in SVs cultured without DHT. The addition of suramin (0.2 mM) to medium containing DHT inhibited the formation of epithelial branches almost completely. Removal of suramin from the medium on days 2, 4, and 6 of culture initiated the formation of epithelial branches. Suramin (0.2 mM) reversibly decreased 3H-thymidine-labeling indices (3H-LI) of both epithelium and mesenchyme of SVs cultured in medium with DHT plus insulin or DHT alone during 8 days of culture. Suramin also decreased 3H-LI of both epithelium and mesenchyme of SVs cultured in medium with insulin alone. The present study indicates that suramin reversibly inhibits not only androgen-dependent but also androgen-independent growth and ductal branching morphogenesis of neonatal mouse SVs.