Islet amyloid polypeptide (IAPP) is a secretory product of the pancreatic β-cell, which is the primary constituent of the islet amyloid that develops in type II diabetes. To study the role the inherent amyloidogenicity of human IAPP (hIAPP) plays in the formation of islet amyloid deposits and to investigate a possible hormonal role for IAPP, transgenic mice expressing hIAPP were developed. The transgene was composed of a fragment of an hIAPP cDNA linked to the rat insulin II promoter. One line of transgenic mice expressed the transgene and synthesized hIAPP in their pancreatic islets. IAPP-like immunoreactivity in pancreatic extracts and plasma were two- to threefold greater in the transgenic mice compared with nontransgenic control mice. Although plasma concentrations of immunoreactive insulin (IRI) and glucose were equal in transgenic and control mice, the pancreatic content of IRI was nearly twofold greater in the transgenic animals, and proinsulin mRNA was significantly elevated, suggesting increased rates of insulin biosynthesis. Pancreatic samples obtained from transgenic mice up to 19 months of age had no evidence of islet amyloid. These results indicate that an increased level of synthesis of the amyloidogenic hIAPP is not sufficient to cause islet amyloid deposition. However, the increased synthesis and storage of insulin in the islets of the transgenic mice are consistent with either a direct regulatory effect of IAPP on the β-cell or indirect stimulation of insulin production through IAPP-induced insulin resistance.