Glyburide, a second-generation sulfonylurea, is used in the treatment of NIDDM because of its hypoglycemic action. However, the site and mechanism of action of this sulfonylurea remain unclear. We examined the ability of glyburide to enhance insulin's inhibitory effect on glucagon-stimulated hepatic glucose production. The livers of fed male rats were perfused with a Krebs-Henseleit buffer containing washed human red blood cells. After a 60-min control period during which the liver was exposed to both insulin and glucagon (10 microU/ml and 11 pg/ml, respectively), the glucagon concentration was increased to 88 pg/ml in the presence of 0, 10, 40, and 240 microU/ml of insulin. Hepatic glucose output and phosphorylase a activity were monitored during the control and elevated-glucagon periods. The glyburide-infused group received glyburide (1.6 microgram/ml) during both the control and elevated-glucagon periods. As expected, high levels of insulin suppressed glucagon-stimulated glucose production and phosphorylase activation. Insulin at a concentration of 10 microU/ml was unable to suppress glucagon's stimulation of glucose production or its activation of phosphorylase. However, in the presence of glyburide it was able to decrease stimulated hepatic glucose production and phosphorylase activation by 40 and 50% respectively. In the absence of insulin, glyburide was unable to suppress glucagon's glycogenolytic action, suggesting that the drug potentiates insulin's action on the liver rather than exerting an inhibitory effect directly. Insulin at a concentration of 240 microU/ml completely suppressed glucagon action, and glyburide had no additional effect. Therefore, glyburide is able to enhance the sensitivity of the perfused rat liver to insulin without altering maximal insulin responsiveness.