Normal sexual maturation in the human male depends upon a thorough regulation of the intratesticular androgen biosynthesis at various ages. We have systematically analysed the intratesticular steriod metabolic pathways in 11 boys, and 5-15 years, by means of incubation of minor testicular biopsy specimens with 3H-progesterone as substrate. Seven boys had abnormally situated testicles, and four had normally descended testicles. For comparison, 3 rats (10, 15 and 90 days old) were also investigated. Significant steroidogenic activity could be demonstrated in all prepubertal testicular tissue specimens. The steroid metabolic products recovered were identical with those found in a previously reported series of nine adult males (Kjessler & Berg, 1976), i.e., 20a-dihydroprogesterone, 17a-hydroxyprogesterone, 20a,17a-dihydroxyprogesterone, androstenedione and testosterone. No appreciable amounts of 5a-reduced substances were found. The steroid metabolic patterns observed in the present prepubertal and adolescent testicles differed from those found in adult males especially by their dissimilar proportions of newly synthesized 20a-dihydroprogesterone and 17a-hydroxyprogesterone. In prepubertal testicular tissue below the age of 11, 20a-dihydroprogesterone accounted for 45-82 per cent, and 17a-hydroxyprogesterone for only 3-6(-17) per cent of all metabolites formed. The steroid metabolic patterns displayed by abnormally situated testicles did not differ from those of normally descended testicles. The variation in steroid metabolic patterns observed between prepubertal and adult testicular tissue may be explained by the existing difference in physiologic gonadotrophic stimulation. It It may also be the result of an age-dependent maturation in various enzyme systems. The regulation of androgen biosynthesis with increasing age in the human male gonads does not seem to involve any significant reduction by 5a-reductase of continuously produced androgens, as in the immature rat. 2nstead, it appears to be brought about by a gradual shift in the metabolic pathways preferred by means of an increased activity of 17a-hydroxylase.