Isomeric methoxyhydroxy metabolites (2) of propranolol were identified in man during chronic antihypertensive propranolol therapy. The identification was based on the mass-spectral fragmentation pattern of their trifluoroacetylated derivatives. Both isomers were detected in urine and in plasma, mainly as glucuronic acid and/or sulfate conjugates. The amounts of these metabolites excreted in urine were linearly related (r = 0.84) to the oral propranolol dose, 10-320 mg/day. Plasma concentrations at the 320-mg daily dose ranged from 20-112 ng/ml. Similar isomeric methoxyhydroxy metabolites of propranolol containing a glycol or a lactic acid side-chain were also identified.