A high-performance liquid chromatographic assay has been developed for the determination of nizatidine, a new histamine H2-receptor antagonist, and two of its main metabolites, N-desmethylnizatidine and nizatidine sulphoxide. Drugs were extracted with chloroform-2-propanol (90:10, v/v) from alkalinized samples of serum, using ranitidine as an internal standard. After evaporation of the extraction solvent, the residue was removed and analysed on a LiChrosorb Si60 5-microns column with a mobile phase of acetonitrile-methanol-water-ammonia solution (1000:200:20:5, v/v). The compounds were detected at 320 nm. The lower detection limits were 6-18 ng/ml at a signal-to-noise ratio of 3. This method is simple and specific, and the single-step extraction makes it rapid. It is the first high-performance liquid chromatographic assay to be described for the determination of nizatidine metabolites.