Whole human testicular tissue, isolated seminiferous tubules and interstitial tissue were incubated with labeled testosterone. Metabolites were isolated by paper and thin-layer chromatography and derivative formation. Results were expressed as percentage of substrate added at the beginning of the incubation, losses being corrected with a second isotope. In the seminiferous tubules, metabolic activity was very poor in prepubertal testes but increased in adolescent subjects. Among the steroids looked for, 5α-androstan-3α,17β-diol and dihydrotestosterone were the main metabolic products isolated in the tubules. Maximum metabolic activity was observed at late meiotic and early post-meiotic stages of development of the germinal epithelium, but the cells bearing this enzymatic activity were not identified. Androst-4-en-3,17-dione was the only product found in the interstitial tissue, but only in testes having mature Leydig cells. These findings were similar to those previously reported in rats. Cryptorchid testes from 2 adolescents with poor development of the germinal epithelium, showed low metabolic activity in the seminiferous tubules but active formation of androst-4-en-3, 17-dione in the interstitial tissue. Reducing activity was present in the testis of a patient with testicular feminization. No activity could be detected in another subject with Klinefelter syndrome.