Intestinal ulcerations, similar to those reported in humans, were produced in the rhesus monkey within 5 days by the administration of 1000 mg of potassium chloride in enteric-coated tablets twice daily. Doses of 250 mg of potassium chloride on the same schedule produced milder lesions which could not be predictably reproduced. Tablets of 100 mg were without effect. The dimensions of the tablet were not the determining factor in the production of lesions. The specific locations of lesions appeared to coincide with the areas where potassium chloride was released or came into contact with the gastric or intestinal mucosa. Various solutions of potassium salts produced erythema and ulcerations of the stomach. A number of commercially available enteric-coated tablets containing various concentrations of potassium chloride and thiazide diuretic produced intestinal lesions ranging from hyperemia to perforating ulcerations. Thiazide diuretics did not influence the severity of the pathology noted nor cause lesions when given alone.