The pharmacokinetics of N1-acetylsulphamethoxazole and N4-acetylsulphamethoxazole in man are described. N1-Acetylsulphamethoxazole is deacetylated to sulphamethoxazole and acetylated to N4-acetylsulphamethoxazole. N4-Acetylsulphamethoxazole is excreted almost unchanged in the urine. The renal excretion rate is independent of the urine flow and urinary pH. N4-Acetylsulphonamides are less lipid soluble and more acidic than their corresponding parent sulphonamides.