A high-performance liquid chromatographic method for the determination of the antithyroid drug propylthiouracil in dog plasma has been developed. Propylthiouracil and the internal standard, methylthiouracil, are extracted from plasma with methylene chloride at pH 6 and the organic layer is evaporated to dryness. The residue is chromatographed on a Chromspher C18 reversed-phase column using Pic B-7 (0.005 M 1-heptanesulphonic acid in water)-1% acetic acid-methanol (40:45:15, v/v/v) as the mobile phase. Quantification is achieved by monitoring the UV absorbance at 300 nm. The response is linear (0.1-15 micrograms/ml) and using 100 microliters of plasma the detection limit is 50 ng/ml. The within-run coefficient of variation is less than or equal to 5% and the accuracy is within 10% of the theoretical value at concentrations between 0.1 and 15 micrograms/ml plasma.