The temporal response patterns of the concentrations of serum testosterone, oestradiol, 17-hydroxyprogesterone, pregnenolone, progesterone, androstenedione and 5 alpha-dihydrotestosterone to a single i.m. dose of hCG (5000 IU/1.7 m2) were investigated in prepubertal and early pubertal cryptorchid boys, and compared with the response patterns obtained earlier in adult men. The rapid response (at approximately 2-4 h) of serum testosterone was lacking in all boys, whereas the slow response at 2-5 days was constant. The relative response (the maximum stimulated concentration vs. the basal level) of serum testosterone was 70-fold in prepubertal boys and 6-fold at early puberty, compared with 2.4-fold in adult men. Serum oestradiol and 17-hydroxyprogesterone concentrations did not increase in the prepubertal boys, but did increase at early puberty, revealing a pattern similar to that observed in adult men. Hence, the prepubertal endocrine testis appears to be very responsive to hCG stimulation, and this responsiveness is rapidly lost with advancing puberty. The absolute increases, however, were smallest in prepubertal boys, perhaps reflecting the small potential Leydig cell mass. The responses of serum oestradiol and 17-hydroxyprogesterone to hCG appeared later during the boys' development than the response of serum testosterone. The relative testosterone response was maximal in the absence of an oestradiol response. It is suggested that testicular oestradiol production in response to LH/hCG appears in the course of puberty and results in intratesticular short-loop feed-back inhibition of androgen production. This is reflected by the appearance of a 17-hydroxyprogesterone response and by a decrease in relative testosterone response.