The percutaneous absorption and excretion of 1% and 5% solutions of minoxidil labeled with carbon 14 were measured in 12 adult male subjects. These subjects were randomly assigned to the 1% and 5% minoxidil testing groups, and all received nine topical applications to a bald area on the scalp, with the radioactive solutions applied on days 1 and 9. Urinary excretion of radioactivity was low, with mean values ranging from 1.6% to 3.9% of applied dose. No radioactivity could be detected in fecal samples. Recovery of radioactivity from the skin surface and from scalp and pillowcase washes was in the range of 41% to 45% of applied dose. No adverse reactions or notable abnormalities were noted in the subjects during the study. Although minoxidil is poorly absorbed through the skin, systemic doses in the range of 2.4 to 5.4 mg/day can be anticipated if application is made to entire scalp.