The transepidermal penetration and metabolic disposition of 14C-benzoyl peroxide were assessed in vitro (excised human skin) and in vivo (rhesus monkey). In vitro, the benzoyl peroxide penetrated into the skin, through the stratum corneum or the follicular openings, or both, and was recovered on the dermal side as benzoic acid. In vivo, benzoic acid was recovered from urine in amounts equivalent to 45% and 98% of the radiolabel following, respectively, topical and intramuscular administration of small amounts of 14C-benzoyl peroxide. We conclude that benzoyl peroxide penetrates as such into the skin layers and is converted therein to benzoic acid, which, in turn is absorbed into the systemic circulation. Renal clearance of the metabolite is sufficiently rapid as to preclude its hepatic conjugation with glycine, since following topical administration to rhesus monkeys, no hippuric acid was found in the urine, as could have been expected had a significant amount of benzoic acid passed through the liver.