The purpose of this study was to investigate the kinetics of urinary excretion of heroin and its metabolites in human subjects. Heroin and its metabolites were determined with gas-liquid chromatography. Two studies were conducted, each using 10 subjects. After i.v. administration of heroin HC1, 10 mg/70 kg, urine was collected every 8 hours and ad libitum for 1 week in the first study and every 2 hours in the first 8 hours and then at less frequent intervals for 24 hours in the second study. Heroin, 6-acetylmorphine, morphine, the sum of conjugates (morphine plus 6-acetylmorphine) and total normorphine were determined in the first 24-hour urine and accounted for 0.5, 1.5, 7.2, 52 and 4%, respectively, of the administered dose. Conjugated morphine could be detected in the urine 96 hours after drug administration. Eighty-eight percent of the free morphine and 84% of the total morphine found in the urine were excreted in the first 8 hours. The half-lives of urinary excretion of free morphine, 6-acetylmorphine, the sum of conjugates (morphine plus 6-acetylmorphine) and total normorphine were 1.28, 1.31, 2.76 and 2.72 hours, respectively. It was concluded that heroin in the body was rapidly metabolized and its metabolites were rapidly excreted in the urine.