The effects of glibenclamide and glipizide on the concentrations of S-glucose, S-insulin and S-lipids and on the 24-hour urinary glucose excretion were studied in 37 patients with maturity onset diabetes. A double-blind, cross over double-dummy technique was used. The fasting S-insulin concentration was higher during glibenclamide therapy, while the increase in insulin concentration one hour postprandially was stronger during glipizide therapy, supporting the concept that glibenclamide has a more prolonged and glipizide a more fast-acting effect on insulin secretion. The S-glucose concentration was lower in the fasting state as well as one hour postprandially during glibenclamide therapy which, together with a lower 24-hour urinary glucose excretion, indicates that glibenclamide has a stronger blood glucose-lowering effect. Although statistically significant, the differences were marginal from a clinical point of view. The lipid levels remained unchanged.