The purpose of the present study was to compare the glucose dependency of the insulin secretagogue activity of the sulfonylurea, glyburide, versus that of glucagon-like peptide-1(7-37) [GLP-1(7-37)] in vitro and in vivo. In freshly isolated rat islets, maximally effective concentrations of glyburide (10 micromol/L) and GLP-1(7-37) (10 nmol/L) were equally effective in stimulating insulin secretion in the presence of 15 mmol/L glucose (2.4-fold increase relative to 15 nmol/L glucose alone). At 5 nmol/L glucose, both agents increased insulin secretion, but the effect for glyburide was threefold greater than for GLP-1(7-37) (122% and 41% increase in insulin secretion, respectively). In conscious catheterized rats infused with glucose at a variable rate to clamp plasma glucose concentration at 11 mmol/L, glyburide (1 mg/kg orally) and GLP -1(7-37) (infused intravenously [IV] at 5 pmol/min/kg) produced similar increase in insulin levels (1.8-fold relative to the respective vehicle controls) that were sustained through 60 minutes of measurement. These doses of GLP-1(7-37) and glyburide were then administered to fasted and fed rats (basal plasma glucose concentration, 5.8 and 7.3 mmol/L, respectively). Relative to the vehicle control group, GLP-1(7-37) infusion produced a transitory increase (30%) in plasma insulin concentration and a modest sustained decrease (10% to 20%) in glucose in both fasted and fed rats, whereas glyburide induced a sustained 2.4- and 1.7-fold increase in plasma insulin concentration in fasted and fed rats, respectively, and a 50% decrease in plasma glucose in both fasted and fed rats. Results of these studies demonstrate the higher glucose threshold for the insulin secretagogue activity of GLP-1(7-37) relative to glyburide in vitro and in vivo.