The amount of free and acetylated norsulfazol in the urine of patients with gastrointestinal diseases was analysed 3 hours after peroral administration of the drug (20 mg/kg). The acetylation rate of norsulfazol in patients of 21--30 years of age was significantly lower than that in heathy subjects of the same age. In groups of 31--40, 41--50 and 51--60 years of age the acetylation rate was 47, 42 and 43%, respectively. In patients with chronic cholecystitis excretion with urine of free norsulfazol was 2-fold as increased whereas in chronic gastritis there was a 2.7-fold increase as compared to controls. The acetylation rate of norsulfazole was 1.6, 1.4 and 1.5-fold reduced in duodenal ulcer, cholecystitis and gastritis, respectively.