1 The disposition and metabolism of labetalol and either 14C- or 3H-labetalol has been studied in mouse, rat, rabbit, dog and man. 2 Radiolabelled labetalol was administered orally at doses of 100 mg/kg to the mouse, up to 50 mg/kg to the rat and rabbit, 20 mg/kg to the dog and 200 mg to man. From measurements of the total plasma radioactivity it was shown that labetalol was well absorbed by all the species. When the measurements of plasma radioactivity and labetalol concentrations were compared, it was found labetaol had been extensively metabolized by the first-pass effect in rat, rabbit and man. Metabolism by this route occurred to a lesser extent in the dog. 3 Radiochemical analysis of the tissues from rats, rabbits and dogs showed that the highest concentrations of radioactivity were found in the lung, liver and kidney. Very little radioactivity was present in the brain. Over 99% of the radioactivity was cleared from the tissues by 7 d. When doses of up to 200 mg 14C-labetalol/kg were given to pregnant rats and 50 mg 14C-labetalol/kg to pregnant rabbits, autoradiographic and radiochemical analysis of the full-term foetuses showed that only small amounts of radioactivity were present in the foetus. 4 The mouse excreted 72%, the rate 48%, the rabbit 61%, the dog 66% and man 60% of the oral dose of radioactivity in the urine. Analysis of themouse and rat faeces showed that the remainder of the dose of radioactivity was excreted in the faeces. 5 Radiochemical analysis of the urine and faeces collected from rats and dogs after an intravenous dose of 1 mg 14C- and 3H-labetalol/kg showed that excretion of radioactivity occurred via both the kidney and bile. 6 An intravenous dose of 1 mg 3H-labetalol/kg to dog and 1 mg 'cold' labetalol/kg to man was not as extensively metabolized as a similar oral dose. 7 The percentage of the dose excreted in the urine as unchanged drug was 2% in the rabbit, 11% in the rat, 19% in the dog and up to 5% in man. The O-phenyl glucuronide was the major metabolite present in the mouse, rat, and rabbit urine. Dog and man also formed this metabolite, but to a lesser extent. A second glucuronide was the major metabolite present in dog urine. This was probably formed through conjugation of glucuronic acid was the secondary alcohol group of labetalol. The major metabolite present in human urine was an unidentified conjugate of labetalol. Minor metabolites of labetalol present in rat, rabbit and dog urine were hydroxylabetalol and its glucuronyl conjugate.