Five maturity-onset diabetics have been studied during therapy with glibenclamide 2.5 mg and 5 mg by half-hourly blood sampling for twelve hours. All patients had lower mean blood glucose concentrations during therapy with 5 mg glibenclamide. There was no significant difference between serum insulin concentrations on the two doses, however, serum insulin/blood glucose ratio was higher during the larger dose of glibenclamide. Mean blood lactate, pyruvate and serum triglycerides were significantly lower, and blood glycerol, 3-hydroxybutyrate, and plasma non-esterified fatty acids were increased during therapy with 5 mg. In the individual patient the changes in blood glycerol and plasma non-esterified fatty acids were related to changes in circulating insulin concentration and did not appear to be a true extra-pancreatic effect of glibenclamide. The mechanism of any extra-pancreatic effect remains unclear.