Prohormone convertase 2 (PC2) plays an essential role in the processing of proglucagon to mature active glucagon in pancreatic α-cells (J Biol Chem 276:27197–27202, 2001). Mice lacking PC2 demonstrate multiple defects, including chronic mild hypoglycemia and dramatic hyperplasia of the pancreatic α-cells. To define the contribution of mature glucagon deficiency to the hypoglycemia and α-cell hyperplasia, we have attempted to correct the defects by delivery of exogenous glucagon by micro-osmotic pumps. Intraperitoneal delivery of 0.5 μg glucagon/h in PC2^−/− mice resulted in the normalization of blood glucose concentrations. Islet remodeling through the loss of hyperplastic α-cells was evident by day 11 after pump implantation; by 25 days postimplantation, PC2^−/− islets were indistinguishable from wild-type islets. These rapid changes were brought about by induction of apoptosis in the α-cell population. Morphological normalization of islets was also accompanied by marked downregulation of endogenous preproglucagon gene expression, but with little or no change in the level of preproinsulin gene expression. Exogenous glucagon delivery also normalized hepatic expression of the gluconeogenic enzyme PEPCK. These results demonstrate that the lack of mature glucagon in PC2^−/− mice is responsible for the aberrant blood glucose levels, islet morphology, and gene expression, and they confirm the role of glucagon as a tonic insulin antagonist in regulating glycemia.