Glucose homeostasis in mammals is maintained by insulin secretion from the β-cells of the islets of Langerhans. Type 2 diabetes results either from primary β-cell failure alone and/or a failure to secrete enough insulin to overcome insulin resistance. Here, we show that continuous infusion of glucagon-like peptide-1 (7–36) (GLP-1; an insulinotropic agent), to young and old animals, had effects on the β-cell of the pancreas other than simply on the insulin secretory apparatus. Our previous studies on a rodent model of glucose intolerance, the aging Wistar rat, show that a plateau in islet size, insulin content, and β-cell mass is reached at 13 months, despite a continuing increase in body weight. Continuous sc infusion of GLP-1 (1.5 pm/kg·min), over 5 days, resulted in normal glucose tolerance. Our current results in both young and old rats demonstrate that treatment caused an up-regulation of pancreatic-duodenum homeobox-1 (PDX-1) expression in islets and total pancreas, induced pancreatic cell proliferation, and β-cell neogenesis. The effects on levels of PDX-1 messenger RNA were abrogated by simultaneous infusion of Exendin (9–39), a specific antagonist of GLP-1. PDX-1 protein levels increased 4-fold in whole pancreata and 6-fold in islets in response to treatment.β -cell mass increased to 7.2 ± 0.58 from 4.88 ± 0.38 mg, treated vs. control, respectively, P< 0.02. Total pancreatic insulin content also increased from 0.55± 0.02 to 1.32 ± 0.11 μg/mg total pancreatic protein. Therefore, GLP-1 would seem to be a unique therapy that can stimulate pancreatic cell proliferation and β-cell differentiation in the pancreas of rodents.