The metabolic clearance rate (MCR) of androsta-4,16-dien-3-one (androstadienone) and the cumulative specific activity (CSA) of urinary 3α-hydroxy-5α-androst-16-ene (androstenol) were measured in two healthy men and in one healthy woman after single intravenous injections of 4-[14C]- or 7α-[3H]androstadienone. Approximate plasma production rates, calculated from the MCR and values for the endogenous androstadienone concentration in plasma pools from six male and six female donors (0·984 and 0·366 μg/l respectively), were 975 and 1341 μg/24 h for the two men and 456 μg/24 h for the woman. The 'production rates' of androstadienone calculated from the CSA of urinary androstenol were 5210, 3730 and 1070 μg/24 h respectively. It follows that urinary androstenol is not uniquely derived from circulating androstadienone and it is concluded that both steroids are secretory products. Alternative sources and precursors of urinary androstenol are discussed. Secretion of radioisotope in the sweat appeared to occur predominantly in the axillae, but was quantitatively no greater than in previous experiments with labelled progesterone and 5-pregnenolone.