The induction of the prostate gland by testosterone or dihydrotestosterone was studied in urogenital sinuses from Wistar rat embryos explanted in organ culture. In normal development, prostatic rudiments appeared in the urogenital sinuses of 18-19 day male foetuses as epithelial buds projecting into the mesenchyme. Male urogenital sinuses from 17-5 and 18-5 day foetuses formed buds in control medium, and addition of testosterone or dihydrotestosterone increased their number and size. In contrast, sinuses from 15-5 and 16-5 day male foetuses did not form buds in the absence of androgens, while exposure to the hormones induced them de novo. A concentration of 0-0015 microgram testosterone/ml was sufficient to elicit prostatic buds. The same effect was seen after continuous treatment with testosterone or brief exposure to the hormone followed by cultivation in control medium. The development of the buds followed the same time sequence as in the organ in situ. Testosterone and dihydrotestosterone elicited prostatic buds in female sinuses from 15-5 to 18-5 day foetuses, but their response to androgens decreased with advancing foetal age. Male and female sinuses from 15-5 day foetuses did not respond to testosterone but formed buds after exposure to dihydrotestosterone. This difference was considered to be due to a low content or absence of 5alpha-reductase at this stage. Testes from 15-5 to 18-5 day foetuses grown in close contact with urogenital sinuses or explants of mature prostate glands induced prostatic buds or augmented the height of the prostatic epithelium. It was concluded that androgens are necessary for the initiation of the rat prostate gland and that the effect of testosterone is mediated via 5alpha-dihydrotestosterone. The onset of 5alpha-reductase synthesis is likely to occur between days 14 and 15 of foetal life. The early androgenic activity of the testes suggests that bud formation in the older male sinuses in the absence of androgens is due to exposure to endogenous testosterone before explanation.