A high performance liquid chromatography method for the determination of sulphamethoxazole and its metabolite N4-acetylsulphamethoxazole is described. The renal excretion rate and cumulative renal excretion of sulphamethoxazole is markedly influenced by urinary pH. With constant urinary pH, the renal excretion rate and the renal clearance of sulphamethoxazole is dependent on the urine flow. The renal clearance of the metabolite N4-acetylsulphamethoxazole is not influenced by urinary pH or urine flow.No clear acetylator phenotype could be detected in the group of volunteers studied. The extent of acetylation depends on the amount of sulphamethoxazole available for acetylation, thus indirectly on the urine pH and flow.