Testosterone (17 beta -hydroxy-4-androsten-3-one ; T) and dihydrotestosterone (17 beta -hydroxy-5 alpha -androstan-3-one ; DHT) concentrations were determined by radioimmunoassay in different fetal tissues taken from male guinea-pigs. Androgen concentrations were maximal in the components of the Wolffian duct (vas deferens, epididymis, seminal vesicle) and the urogenital sinus (urogenital tubercle, prostate) when these tissues are differentiating between days 28 and 36 (T = 320 to 1450 and DHT = 200 to 860 pg/10 mg of tissue). During the same period circulating testosterone is taken up by the non-specific tissues (intestine, diaphragm) to a lesser degree (150 to 250 pg/10 mg) as well as by hypothalamus and hypophysis (100 to 170 pg/organ). After this uptake phase, T declines in the non-specific tissues to minimal concentrations (less than 10 pg/10 mg). Compared to the situations in the diaphragm and intestine, DHT concentrations were significantly higher in both urogenital sinus and Wolffian duct components, and T concentrations were significantly higher only in the Wolffian ducts components. In the bladder, T and DHT levels were significantly higher than those of the androgen-independent tissues.