Hydrochlorothiazide, a nonmercurial diuretic agent with the chemical formula 6-chloro-7-sulfamyl-3,4-dihydro-1,2,4-benzothiadiazine-1,1-dioxide, is clinically effective when given orally or intravenously.1 It is approximately 10 times as potent as chlorothiazide. In a previous study,2 chlorothiazide labeled with radioactive carbon (C14) was used to study the disposition of this drug in human beings. The results suggested that the amount of radioactivity recovered in the urine was in inverse proportion to the severity of the cardiac, hepatic, or renal disease existing in the patient. In the present study, C14-labeled hydrochlorothiazide was used as a tracer to study the behavior of hydrochlorothiazide in normal subjects and in patients with various diseases. The effectiveness of this substance was compared with that of its parent compound, chlorothiazide. Material and Methods Material.— A total of 19 studies were performed in 15 subjects—4 men and 11 women—ranging in age from 21 to 70 years. The