In four children with vasopressin-sensitive diabetes insipidus, the hypoglycemic agent chlorpropamide caused a decrease in urinary flow rate and free water clearance without change in osmolar clearance. Only two children, however, had a satisfactory response. The effect of chlorpropamide resembles that of vasopressin injection. One child with nephrogenic diabetes insipidus showed no response. Symptoms of hypoglycemia were observed in all children. Blood glucose determinations were in the range of 53 to 75 mg/100 ml but were found as low as 16 mg/100 ml. In two patients, intravenous glucose administration was necessary to sustain a normal blood glucose level. The risk of hypoglycemia limits greatly the use of chlorpropamide in the treatment of diabetes insipidus in children. It appears to be a more frequent and greater hazard in children than in adults.