A high-performance liquid chromatographic method has been developed for the determination of ranitidine in plasma. Ranitidine was extracted with acetonitrile by adding it to the plasma and then salting it out with potassium carbonate. The chromatographic column was 5-microns ODS silica, the mobile phase being acetonitrile-7 mM triethylammonium ion in phosphoric acid (pH 3.00) (30:70, v/v). The ranitidine peak was monitored at a wavelength of 315 nm, the retention time for ranitidine being 4.6 min. A limit of detection of 3 ng ml-1 was obtained for a 100-microliters injection of ranitidine. The method was found to be reproducible with a relative standard deviation (RSD) between 0.8-5.3% (n = 5) over the concentration range 25-80 ng ml-1 in plasma. The ranitidine concentration was determined in 18 different patients' plasmas. Ranitidine and its metabolites ranitidine S-oxide, ranitidine N-oxide and desmethyl-ranitidine, were also studied for chromatographic resolution from each other. It was shown that a group of common drugs did not interfere with ranitidine determination.