The pattern of response to 14-day hCG stimulation was studied in 100 apparently normal infants and children and in 2 boys with total adrenal insufficiency. Response was estimated on the levels (nanograms per dl) of testosterone (T), delta4-androstenedione (delta4), and 17-hydroxyprogesterone (17OHP) before and after hCG. Basal levels of T, delta4, and 17OHP increased significantly after 9--10 yr of age and before the onset of puberty. It would thus appear that the changes of adrenarche are also expressed by the secretion of delta4 androgens. After hCG, levels of T (629 +/- 218) did not vary with age in infants or prepubertal children but increased significantly at the onset of puberty (1265 +/- 253). Post-hCG levels of delta4 and 17OHP increased with age, but their increments (respectively, 31.1 +/- 14.6 and 108.6 +/- 42.2) did not vary between 2--12 yr of age. On the other hand, the rise of delta4 after hCG tended to be higher in infancy and early pubescence. These data suggest that 1) testicular unresponsiveness to gonadotropins does not seem to be responsible for the postnatal decline of testicular activity; 2) during the prepubertal period there are no age-related changes in the absolute rises of T, delta4, or 17OHP levels after hCG stimulation; and 3) increasing sensitivity of the testis to gonadotropic stimulation appears to be one of the very first changes occurring at the onset of puberty.