The influence of age and sex on the peripheral metabolism of testosterone was studied by giving intravenous tracers of 14C-testosterone to 21 prepubertal children (13 boys and 8 girls), 39 young adults 18–43 years old (23 men and 16 women), and 10 elderly adults 68–86 years old (6 men and 4 women). Studies were also carried out in 2 sexually immature young adults, one 18-yearold 45 XO phenotypic female with gonadal agenesis and one 18-year-old 45 XO, 46 XX mosaic female with gonadal dysgenesis; the latter was restudied after prolonged estrogen-progestagen therapy. Age and sex influences were observed only with respect to the androsterone/etiocholanolone (A/E) ratio; a sex difference in diol metabolite formation was not observed. Prepubertal children showed no sex difference in A/E ratio, which averaged 1.7 ± 0.28 in boys and 1.9 ± 0.42 in girls. Young adult men showed a slightly lower A/E ratio, averaging 1.5 ± 0.10, while females showed a much greater decrease in A/E ratio, to 0.9 ± 0.09, so that there was a highly significant (P < .001) sex difference in this age group. The decreased averages were due to disappearance of the higher end of the ranges seen in prepubertal children; the lower limit of the ranges remained the same. Elderly adult men showed a further fall in the A/E ratio, to 1.0 ±0.11, and elderly women also showed a further fall, to 0.4 ± 0.04; a highly significant (P < .005) sex difference remained. Once again, the fall in average A/E ratio from young adults to elderly adults was due to disappearance of the higher end of the ranges in the former; the lower limits of the ranges were the same in both groups. Of the 2 sexually immature young women, one showed an A/E ratio of 1.3, just below the upper limit for young adult women, and the other showed a ratio of 1.8, well above that limit and thus typical of prepubertal girls. Estrogen-progestagen therapy of the second girl decreased the A/E ratio to 1.4, the upper limit for young adult women. It was concluded that there is a fundamental aging effect in both sexes which causes a gradual progressive decrease of the mean A/E ratio as a result of progressive disappearance of the higher individual A/E values while the lower end of the range of values remains constant; superimposed on this gradual decrease is an acute pubertal decrease in females, probably mediated by the development of the estrogen-progestagen milieu characteristic of sexually mature women.