Glucagonlike peptide I (7–37) [GLP-I-(7–37)], encoded with glucagon and glucagonlike peptide II and intervening peptide II in the rat and human glucagon gene, is processed from proglucagon in both pancreas and intestine and is a potent stimulator of insulin secretion. Unequivocal insulin release from the isolated perfused rat pancreas is elicited by a 10⁻¹¹ M concentration of this peptide, and a weak response is found at 10⁻¹² M. We found that GLP-I-(7–37) is ∼100 times more potent than glucagon in the stimulation of insulin secretion. Insulin release in response to GLP-I-(7–37) is highly dependent on the ambient glucose concentration; no response is detectable at a glucose concentration of 2.8 mM, and at 6.6 and 16.7 mM, insulin release is augmented by 4.7 and 22.8 ng/ml, respectively. The pattern of insulin secretion stimulated by GLP-I-(7–37) is biphasic, with an initial spike followed by a plateau of sustained release. The effects on insulin release of GLP-I-(7–36) amide, a GLP-I analogue, and GLP-I-(7–37) at concentrations of 10⁻¹¹ M were indistinguishable. We also found that GLP-I-(7–37) at 10⁻⁹ M does not influence glucagon secretion and that glucagonlike peptide II and the intervening peptide II, two other peptides encoded by the glucagon gene, have no detectable effects on insulin secretion.