Pharmacokinetic studies following the simultaneous treatment of sheep or cattle with the anthelmintics triclabendazole and fenbendazole required an assay for determining the analytes in the plasma. An extraction and clean-up procedure is described which uses a C(18) Sep Pak cartridge, involves no evaporation steps and produces a clean extract from sheep and cattle plasma in which triclabendazole, fenbendazole and their respective oxidation products may be determined by reversed-phase high-performance liquid chromatography. Results from application of the method to a pharmacokinetic study of sheep are presented.