The in vitro metabolism of radiolabeled testosterone was studied in the following human tissues: fetal müllerian ducts, fetal wolffian ducts, fetal testes, fetal and adult skeletal muscle, fetal perineal and control (thigh) skin, adult vas deferens, adult fallopian tubes, and perineal skin and testes from 2 siblings with prepubertal male pseudohermaphroditism. By isolating and identifying all the major metabolites, the predominance of the 17-“oxo” metabolic pathway was demonstrated in all the tissues, except perineal skin, vas deferens and testes. A comparison of the 17-“oxo”/dihydrotestosterone ratios in fetal and adult tissues suggests that female sex steroids may increase metabolism along the 17-oxidative pathway. The greatest conversions to dihydrotestosterone were found in fetal perineal skin, a tissue markedly sensitive to androgens. Dihydrotestosterone formation by the adult vas deferens was significantly greater than by the adult fallopian tubes. However, 5 areductase activity was found in all types of tissues, including testes and skeletal muscle, but not in fetal testes or adult female skeletal muscle.