A high-performance liquid chromatographic (HPLC) method was developed for the determination of diltiazem (DTZ) and its major metabolite, deacetyldiltiazem (DAD), in rat plasma. DTZ, DAD and imipramine, the internal standard, were selectively fractionated from plasma on a C_(18) reversed-phase column (μ-Bondapak, 10 ㎛ silica, 300×3.9 ㎜ ID). The composition of the mobile phase was methanol: acetonitrile: 0.04 M ammonium bromide: triethylamine (40:24:36:0.06 in volume). The pH of the mobile phase of their method was lowered to 6.4. The eluents from the column were detected for DTZ and DAD using a UV detector at 237 ㎚. The recovery was >85% for DTZ and DAD, and average intra-day and inter-day coefficients of variation were <6% for DTZ and DAD at the concentration ranges of 20-1000 ng/㎖. Detection limit of DTZ and DAD in plasma was 20 ng/㎖ with signal-to-noise ratio of 3. This method would be applicable to practical pharmacokinetic studies without detriment to the HPLC column.