Ten healthy, male volunteers (aged 19-27 yr; weight 62-67 kg) were randomly distributed into control and test groups of five subjects each in a controlled study on the effect of ranitidine on chloroquine disposition. The control group subjects received two tablets of chloroquine sulfate (300-mg base) only. The test group subjects received ranitidine 250 mg hs for four days prior to the administration of chloroquine sulfate and throughout the sample collection period. Blood samples (5 ml) were collected from the time of the chloroquine administration to the seventh day after drug administration. The samples were analyzed for chloroquine content by a combination of thin-layer chromatography and ultraviolet spectrophotometry. The Wilcoxon test at 0.05 significance level was used to compare the disposition parameters between control and test groups. Ranitidine therapy was associated with no significant alterations in chloroquine oral clearance rate, elimination rate constant, and apparent volume of distribution. Unlike cimetidine, ranitidine does not interact pharmacokinetically with chloroquine. Ranitidine, therefore, may be the H2-receptor antagonist of choice for ulcer patients receiving chloroquine.