SummaryThe antidiuretic activity of intravenously injected chlorpropamide and some renal functions before and after its administration were investigated in normal persons and in patients with diabetes insipidus.The rapidity of the effect and the significant fall of diuresis and free water clearance indicate a direct action of chlorpropamide on the renal tubute, similar to that of intravenous vasopressin.The drug did not influence blood sugar levels except in one case, with a remarkable weaker antidiuretic effect. Some hypotheses about the antidiuretic action of chlorpropamide are reviewed.