The pharmacokinetics of sulphamethizole, sulphamethoxazole. sulphadiazine, sulphapyridine and sulphadimidine have been studied in man. Renal clearance values of the metabolite N4-acetylsulphonamide are 6 to 20 times higher than those of the corresponding parent compound. The renal clearance of sulphonamides is dependent on the urine flow.N4-Acetylsulphonamide concentration-time profiles for plasma and urine have been constructed for the sulphonamides. The percentage N4-acetylsulphonamide-time profiles for plasma are excellent tools for establishing the acetylator phenotype, while those constructed from urine samples are less useful. Evidence is obtained that sulphadimidine is metabolically processed by 2 different isoenzymes, while sulphadiazine, sulphapyridine and sulphamethoxazole are processed by I acetylating isoenzyme. Sulphamethizole is acetylated to very little extent.