A rapid and sensitive column-switching high performance liquid chromatographic procedures was described for determination of captopril in plasma and urine. p-Bromophenacyl bromide, a derivatizing reagent, was added to the plasma and urine samples to form an adduct which showed ultraviolet absorbing properties. After that, the urine sample was injected directly and for plasma the protein was removed with 6% of perchloride before injection. The column-switching system was equipped with a pre-column of 5 cm x 0.5 cm ID, packed with mu Boundapak C18, 37-50 microns, and an analytical column of 15 cm x 0.5 cm ID, packed with YWG-C18, 10 microns. After washing out the impures from pre-column with 0.2% acetic acid the retained substances were eluted into analytical column with acetonitrile-water-acetic acid (35:65:0.4). Captopril was detected at 260 nm. The method was linear within 20-1000 ng/ml for plasma and 10-200 micrograms/ml for urine. The recovery averaged 103.2% and 99.5% for plasma and urine, respectively. The coefficient variations were all less than 10%.