The relationship between dissolution behaviour and plasma concentration after administration of a sustained-release preparation containing phenylpropanolamine hydrochloride was studied in 11 male volunteers. The testing for dissolution was carried out using the JP X rotating basket procedure, and human plasma samples were assayed for phenylpropanolamine hydrochloride by a specific high-performance liquid chromatography. Three hours after dosing, the mean peak plasma concentration was 104.1 ng/ml and the plasma concentration declined with a half-life of 4.85 h. The time course of plasma and dissolution data were analyzed by a non-linear least squares curve fitting method to compare in vivo and in vitro release behaviour. Plasma concentration changes of phenylpropanolamine hydrochloride were correlated with in vitro release behaviour of the sustained-release preparation.