An assay is described for the simultaneous determination of propranolol and its active metabolite, 4-hydroxypropranolol, in human plasma. Both compounds were separated from an ethereal extract by high-performance liquid chromatography employing a C18 bonded-phase column. Detection of the effluent was by fluorescence. Suitable fluorescent spectrometers and wavelength settings that allow optimum detection of both compounds have been described. The limit of sensitivity was 2 ng/ml for both propranolol and 4-hydroxypropranolol. Mean peak plasma levels of propranolol and 4-hydroxypropranolol in six patients receiving a single dose of a slow-release 160-mg formulation of propranolol were 28 and 6 ng/ml, respectively. These levels were about one-tenth the level obtained following a single conventionally prepared dose of propranolol (160 mg). Peak levels were delayed and plasma levels of propranolol persisted for a longer period with the slow-release formulation. Area under the curve estimates suggested that the bioavailability of the slow-release formulation following single-dose administration was about one-third that of the conventional preparation.