The metabolism of 14C-5-diazepam has been studied in 5 patients with T tube biliary drainage. A single bolus of 40 to 50 μCi was given intravenously and blood, urine, and bile were analyzed from 5 to 14 days. The mean half-life of elimination from blood was 93.2 hr; the major metabolite noted in blood was N-desmethyl-diazepam. in urine the average recovery of radioactivity was 48.9% and consisted of 3 OH-diazepam, 4'OH-diazepam, and oxazepam. in bile the average recovery of radioactivity was 5.35% (corrected to a bile flow of 700 ml was 15.0%) and consisted of the same metabolites as in the urine. Essentially no diazepam or N-desmethyl-diazepam was found, and therefore an enterohepatic circulation cannot be held to account for the prolonged half-life of these substances in man.