The cimetidine concentration of serum and urine was measured by high-performance liquid chromatography (HPLC). Using procaine hydrochloride as an internal standard, samples were applied to a μ-Bondapak C(18) column with acetonitrile/10 mM potassium phosphate buffer, pH 4.8, (7/93 by volume) as the mobile phase. The recovery with this method was 62.6% for serum and 65.0% for urine. The quantitative determination of cimetidine in serum and urine can be performed from 0.1 mg/l to 500 mg/l and from 6.25 mg/l to 2000 mg/l, respectively. For this determination, only 200 μl of serum or urine were required. The column for separation of cimetidine was found to be stable for at least 8 months (1,300 samples). This simple method is suitable for routine measurement of cimetidine in serum and urine.