The effect of three sulfonylurea drugs, acetohexamide, tolazamide, and glyburide, on water excretion in hydrated normal subjects was investigated and compared with that of chlorpropamide. The antidiuretic action of chlorpropamide was confirmed, whereas acetohexamide, tolazamide, and glyburide enhanced water excretion; the latter drug also enhanced the excretion of water by patients with diabetes mellitus. Glyburide did not change excretion of total solute, sodium, potassium, and did not alter plasma 11-hydroxycorticoid concentration. Glyburide did not suppress the excretion of antidiuretic hormone and did not inhibit the antidiuretic action of infused antidiuretic hormone. Therefore, the diuretic action of glyburide does not appear to be mediated through effects on solute excretion, activation of the adrenal cortex, or on release or action of antidiuretic hormone. The effect of the sulfonylurea drugs on water metabolism should seriously be considered when prescribing oral hypoglycemic agents, especially for patients with diabetes mellitus who have a water-retaining tendency.