The pharmacodynamics of propranolol were studied in patients with renal functional impairment. 14C-labelled propranolol was given either intravenously or by mouth and the disappearance rates of propranolol, 4-hydroxypropranolol, and total radioactive metabolites measured. The renal clearance of total radioactive compounds is directly related to renal function. The half-life of total radioactivity is greatly lengthened in the presence of severe renal failure while the half-lives of the pharmacologically active propranolol and 4-hydroxymetabolite are slightly reduced. There is a suggestion that the absorption of propranolol is delayed in renal failure. No known pharmacological action or side effects from the other metabolic products of propranolol have been recognized. There is still too little well-documented evidence concerning the beta-blocking activity of the unidentified major metabolites of propranolol to suggest any alteration in the dosage regimen used in renal failure.