A simple and sensitive reversed-phase high-performance liquid chromatographic method for the determination of diltiazem in human serum has been developed. The method involves a one-step deproteinization of serum for sample clean-up using acetonitrile. A LiChrosorb RP-8 column (30 cm x 4.1 mm I.D.) was eluted isocratically with acetonitrile-0.01 M dibasic sodium phosphate (40:60, v/v) containing 0.01% triethanolamine. Diltiazem was monitored at 237 nm and 0.1 a.u.f.s. The completion time for assay was less than 15 min, and the lower limit of quantitation was 10 ng/ml for a 100-microliters injection volume. Using this method, the pharmacokinetic parameters were calculated from a serum concentration versus time profile of diltiazem.