SummaryTwo cases of hyponatraemia with clinical symptoms of water intoxication are reported in diabetic patients treated with oral antidiabetic drugs. Hyponatraemia induced by chlorpropamide is actually well established. In the first case presented, the role of chlorpropamide in the development of hyponatraemia is confirmed by a therapeutic test. A review of the literature about the mode of action of this sulfonylurea is presented.The second patient developed severe hyponatraemia with water retention after treatment with both saliuretic and biguanide (phenformin). Although in this second case the therapeutic test was less definite, it seems very likely that phenformin played a role in the development of the hyponatraemia. Indeed, an antidiuretic action of biguanides has been previously reported in patients with diabetes insipidus.