Glucagon-like peptide-1 (GLP-1) regulates energy intake, gastrointestinal motility, and nutrient disposal. The relative importance of the islet β-cell for GLP-1 actions remains unclear. We determined the role of the islet β-cell and the pancreatic duodenal homeobox-1 (Pdx1) transcription factor for GLP-1 receptor (GLP-1R)-dependent actions through analysis of mice with β-cell-specific inactivation of the Pdx1 gene (β-cell Pdx1-/- mice). The GLP-1R agonist exendin-4 (Ex-4) reduced glycemic excursion following intraperitoneal (i.p.) glucose challenge in control littermates (β-cellPdx1+/+ mice) but not in β-cell Pdx1-/- mice. Similarly, Ex-4 failed to increase levels of plasma insulin, pancreatic insulin content, and pancreatic insulin mRNA transcripts in β-cellPdx1-/- mice. Furthermore, Ex-4 significantly increased β-cell proliferation and reduced β-cell apoptosis in β-cell Pdx1+/+ mice but not in β-cellPdx1-/- mice. Moreover, Ex-4 increased the levels of insulin and amylin mRNA transcripts and augmented glucose-stimulated insulin secretion in islets from β-cell Pdx1+/+ mice but not in β-cellPdx1-/- islets. Surprisingly, Ex-4 failed to reduce levels of plasma glucagon in β-cellPdx1-/- mice. These findings demonstrate that Pdx1 expression is essential for integrating GLP-1R-dependent signals regulating α-cell glucagon secretion and for the growth, differentiated function, and survival of islet β-cells.