Testicular testosterone reserve capacity of 6-15 yr old boys was estimated after operative correction of testicular maldescensus by maximal stimulation test. Subnormal plasma testosterone levels were found in only 2 out of 14 patients with bilateral and 4 with unilateral orchidopexy. Prepubertal boys with unilateral anorchia had normal basal testosterone values and a normal testosterone rise after stimulation. In prepubertal boys with bilateral testes atrophy there was observed a diminished rise after stimulation. Basal testosterone levels were normal. Basal testosterone levels of pubertal boys with unilateral anorchia or bilateral atrophy were subnormal, and the stimulation of testosterone production was reduced. The testicular volume of patients without atrophy or anorchia after orchidopexy was normal in prepuberty. During puberty a progressive relative decrease of the testicular volume was observed as compared to normal development. The endocrine function in most patients with unilateral or bilateral orchidopexy is in the normal range; a regular puberty can be expected.