The concentrations of testosterone and dihydrotestosterone was determined in the human testis at different stages of sexual development, in patients with undescended testis or in subjects who had been treated for acute lymphoblastic leukemia. Patients were divided in three groups, prepubertal (Tanner's stage I), early puberty (Tanner's stages 2 + 3) and late puberty (Tanner's stages 4 + 5). Even though tubular diameter and serum testosterone followed the expected gradual rise during sexual maturation, testicular testosterone increased abruptly at early puberty, resulting in a high testicular: serum testosterone ratio at this stage. The testicular dihydrotestosterone:testosterone ratio was also high at early puberty. These data suggest that, at initiation of sexual development, seminiferous tubules receive a high androgen load, at a time when other androgen target organs are barely stimulated.