An assay is described for the determination of the H2-receptor antagonist, cimetidine, in human plasma and urine. Alkalinized plasma or urine was extracted with methylene chloride, the organic phase was evaporated, and the reconstituted residue was analysed by high-pressure liquid chromatography (HPLC) using a reversed-phase prepacked plastic column housed in a radial compression module. The metabolite, cimetidine sulfoxide, was identified but could not be quantitated due to interference from the solvent front. The sensitivity limit of the assay was 25 ng/ml. The assay was applied to the measurement of plasma and urine samples in a pilot pharmacokinetic study. Cimetidine was substantially absorbed and rapidly eliminated (plasma elimination half-life of 112-130 min). Plasma cimetidine concentrations could be measured to 12 hr after a 200-mg dose (iv or oral), but they were below the sensitivity of the assay by 24 hr. Urinary excretion of unmetabolized cimetidine accounted for 40-50% of the administered dose in the first 12 hr. This assay is simpler and more sensitive than those previously described, and it is suitable for the measurement of cimetidine in plasma and urine of subjects receiving doses appropriate for clinical use.