An assay for the determination of cimetidine in human plasma is described. Cimetidine was extracted from alkalized plasma with ethyl acetate, washed once over hydrochloric acid, re-extracted into ethyl acetate, and the organic phase was evaporated to dryness. The residue was dissolved in ethanol and injected into a liquid chromatograph. In vitro sulphoxidation was found to occur in whole blood, for which reason the assay was performed in plasma. The accuracy of the method was found to be within 3% and the lower limit for sensitivity was demonstrated to be 0.1 mg/l using 750 microliters plasma. Five volunteers received 1 g cimetidine perorally per day given in four doses with various intervals. Blood samples were drawn hourly, five dose intervals over two days. The average minimum concentration of plasma cimetidine was found to correlate significantly with the mean value of the area under the time/concentration curve over a period of three dose intervals (r = 0.96).