A sensitive high-performance liquid chromatographic method was developed for the simultaneous determination of ranitidine and its metabolites, ranitidine N-oxide, ranitidine S-oxide and desmethylranitidine, in human plasma and urine. For the plasma analysis, 1-ml plasma samples spiked with phenylpyramidol as the internal standard were extracted at basic pH with acetonitrile-ethyl acetate (3:2, v/v). After evaporation and reconstitution, the samples were chromatographed on a cation-exchange column, with a mobile phase of 0.1 M sodium acetate buffer (pH 5)-acetonitrile-tetrahydrofuran (56.5:36:7.5, v/v) and ultraviolet detection at 320 nm. The extraction recoveries were 99.8, 30.4, 74.2 and 80.2% and the detection limits were 5, 15, 10 and 4 ng/ml for ranitidine, ranitidine N-oxide, ranitidine S-oxide and desmethylranitidine, respectively. For the urine analysis, a simple deproteinization with an equal volume of acetonitrile was satisfactory for sample preparation. The applicability of this method for the pharmacokinetic study of ranitidine following oral administration was demonstrated.