Insulin secretion from pancreatic beta-cells has to be tightly regulated to ensure accurate glucose homeostasis. The capacity of beta-cells to respond to extracellular stimulation is determined by several signaling pathways. One important feature of these pathways is phosphorylation and subsequent dephosphorylation of a wide range of cellular substrates. Protein phosphatase 1 (PP1) is a major eukaryotic serine/threonine protein phosphatase that controls a multitude of physiological processes. We have investigated the expression and cellular distribution of two endogenous inhibitors of PP1 activity in beta-cells. RT-PCR, Western blotting, and immunohistochemistry showed that DARPP-32 and inhibitor-1 are present in insulin-secreting endocrine beta-cells. Subcellular fractionation of mouse islets revealed that both PP1 inhibitors predominantly localized to cytosol-enriched fractions. Inhibitor-1 was also present in fractions containing plasma membrane-associated proteins. These data indicate a potential role for DARPP-32 and inhibitor-1 in the regulation of PP1 activity in pancreatic beta-cell stimulus-secretion coupling.