Chlorothiazide (Diuril), a nonmercurial diuretic agent with the chemical formula 6-chloro-7- sulfamyl-1,2,4-benzothiadiazine-1,1-dioxide, has been shown clinically to be effective in the treatment of hypertension and various disease states associated with retention of water and electrolytes. 1,2 Because the chemical method for assaying chlorothiazide is too insensitive for accurate measurements, little is known about the metabolism of this compound in human beings following its administration in pharmacologic doses. Beyer, 3 using chlorothiazide tagged with radioactive carbon (C 14 ), has determined plasma and blood levels of the compound in dogs, and has found them comparable to the chlorothiazide levels obtained by colorimetric assay. Preliminary studies on human subjects indicated that C 14 -labeled chlorothiazide is not metabolized to any great extent and that its excretion is rapid; hence it was felt that the compound could be used without danger in human beings. In the present study it was employed in an effort to delineate the behavior of