Serum and testicular concentrations of testosterone were determined by radioimmunoassay in four prepubertal patients (Tanner's genital stage G1) with unilateral cryptorchidism and in ten pubertal patients with different degrees of genital development (stages G2-G5), five with unilateral and five with bilateral cryptorchidism. In every case, testicular testosterone concentration was determined in the cryptorchid gonad. Two prepubertal autopsy cases were also studied. Results were compared with scrotal testes of adult subjects. Pubertal development resulted in a gradual increase in tubule diameter on microscopic examination notwithstanding the abnormal position of the gonad. As expected, serum testosterone also increased gradually throughout sexual development. Testicular testosterone concentration however increased to adult levels at early stages of puberty. Mean concentrations and ranges were: G1, 26 ng/g (range 7-49, n = 6); G2 + G3, 660 (range 37-1357, n = 6); G4 + G5, 438 (range 272-702, n = 4) and adults 520 (range 356-663, n = 4). This resulted in similar testicular: serum testosterone ratios in G1, G4 + G5 and adults but a high ratio in G2 + G3. Patients of groups G2 + G3 did not have elevation of serum gonadotrophins and did not show Leydig cell hyperplasia. Therefore, in early puberty, when peripheral tissues are exposed to low testosterone levels, seminiferous tubules of cryptorchid gonads might already receive high androgen stimulation.