In 39 boys with cryptorchid or ectopic testes, testosterone responses of the testes to a single dose (5.000 IU/m2) of human chorionic gonadotropins (hCG) were evaluated. Plasma testosterone levels before the administration of hCG (day 0) and three days after were compared to the basal concentrations of endogenous follicle-stimulating hormone (FSH) and luteinizing hormone (LH). No differences of testicular testosterone responses were observed between 10 bilateral cryptorchidisms (day 0: 99.0 ± 19.8 ng/100 ml, day 3: 302 ± 37.1, mean ± sem), 12 unilateral cryptorchidisms (day 0: 82.4 ± 16.5, day 3: 277.5 ± 52.2 ng/100 ml), and 12 unilateral ectopic testes (day 0: 64.1 ± 13.9, day 3: 228.3 ± 45.4 ng/100 ml), all in stage P1 of puberty. In 5 boys with unilateral cryptorchid or ectopic testes in stage P2 of puberty, the basal levels of testosterone were higher (200.0 ± 35.5 ng/100 ml) and the responses to hCG greater 712.0 ± 103 ng/100 ml). In all the subjects, basal testosterone concentrations were correlated to the endogenous FSH levels (r = + 0.544, p < 0.001). The same correlation was observed between the testosterone responses of day 3 and the basal FSH levels (r = + 0.501, p < 0.001). No such correlations on day 0 (r = + 0.274, t = 1.713) and day 3 (r =+ 0.261, t = 1.626) were noted between plasma testosterone and LH. As shown by in vitro as well as in vivo studies, although testosterone secretion is mainly controlled by LH, the response of the testes to exogenous hCG is dependent on the levels of endogenous FSH. FSH probably plays an important mediating role on the secretion of testosterone.