A high performance liquid chromatographic (HPLC) method fot the determination of sulphadiazine and trimethoprim in medicated fish feedstock is described. It is based on the extraction of the drugs 2215 Copyright 0 1991 by Marcel Dekker. Inc. Downloaded by [McMaster University] at 16:22 15 June 2016 MC NALLY ET AL.into methanol followed by separation on a )I-Bondapak column using an aqueous acetonitrile mobile phase of pH 3.0. The method was validated by determining intra-and inter-assay precision and the coefficient of variation was found to be less than 8% for both drugs. Callbration curves were linear over the range 160-4000 pg drug per g of feedstock and the method was applied to the determination of a commercial feedstock formulation.