1. Two volunteers given a single oral dose of [3H]oxprenolol, excreted 70% and 95% of the radioactivity in the urine in 48 h. 78% and 71% of the urinary radioactivity is attributed to metabolites more polar than oxprenolol. 2. Metabolites, identified by mass spectrometry and isotope dilution analysis were as follows: I. the glucuronide of unchanged oxprenolol, II. a conjugate of a derivative of oxprenolol hydroxylated in the aromatic ring, III. unchanged oxprenolol, IV. the hydroxycarboxylic acid derived from oxprenolol by oxidative deamination, V. a carboxylic acid with the carboxyl group in the position of the former carbinol, VI. the monoallyl ether of catechol.