Summary. The concentrations of testosterone and dihydrotestosterone (DHT) were measured in the testis and in different segments of the epididymis and vas deferens of adult mice. There were marked regional variations in the concentrations of testosterone and DHT from the testis to the caudal part of the vas deferens. In the testis, testosterone was the predominant androgen (364 ± 90 ng/g) while DHT was weakly represented (8 ± 2 ng/g). Qualitative and quantitative changes occurred in epididymis: DHT was the main steroid in the caput (29·3 ± 2·7 ng/g) and corpus (33·1 ± 4·4 ng/g) while testosterone and DHT were in similar quantities in the cauda (18·6 ± 2·6 and 19·0 ± 2·7 ng/g, respectively). The proximal region of the vas deferens contained higher amounts (71·4 ± 8·0ng/g) of androgens (testosterone + DHT) than did the caput epididymidis (39·1 ± 3·3 ng/g). Testosterone was the predominant androgen in each part of the vas deferens and its concentrations decreased from the proximal (64·5 ± 7·5 ng/g) to the caudal (26·9 ± 4·3 ng/g) region. Castration and section of the efferent ducts of the testis showed that the epididymis received testosterone essentially via the blood supply and that epididymal DHT was produced locally from circulating testosterone.