The in-vitro metabolism of [3H]testosterone by human scalp and back skin was examined for possible differences in enzyme activity in skin from these two areas, both of which contain large sebaceous glands but only one of which, the back, is prone to develop acne. Punch biopsy specimens of skin, obtained from the scalp and back of adult men, were minced and incubated with [3H]testosterone. The metabolic products were diluted with carrier steroids, then separated and measured by thin-layer chromatography and by gas chromatography on an instrument equipped with a splitter. The results showed that of the 5alpha-reduced metabolites identified, a major one in both the scalp and back skin incubations was 5alpha-androstane-3beta,17beta-diol. Formation of the androstanediol was especially pronounced in scalp skin where it accounted for up to 50% of the 5alpha-reduced metabolites produced. This finding that 5alpha-androstane-3beta,17beta-diol is a major product of testosterone metabolism in vitro by human skin containing sebaceous glands, supports the possibility, previously suggested by studies in the rat, that this steroid can stimulate sebum secretion.