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Several regulatory systems are implicated in the regulation of islet function and [beta] cell mass. Of great interest in this context are some endocrine, paracrine/autocrine, and intracrine regulators. These include, to name but a few, the gut peptides, growth factors, prostaglandins, and some vasoactive mediators such as nitric oxide, bradykinins, endothelins, and angiotensins. Apart from its potent vasoconstrictor actions, the renin-angiotensin system (RAS) that generates angiotensin II has several novel functions-stimulation and inhibition of cell proliferation; induction of apoptosis; generation of reactive oxygen species; regulation of hormone secretion; and proinflammatory and profibrogenic actions. In the pancreas, recent evidence supports the presence of an islet RAS, which is subject to activation by islet transplantation and diabetes. Such a local islet RAS, if activated, may drive islet fibrosis and reduce islet blood flow, oxygen tension, and insulin biosynthesis. Moreover, activation of an islet RAS may drive the synthesis of reactive oxygen species, cause oxidative stress-induced [beta] cell dysfunction and apoptosis, and thus contribute to the islet dysfunction seen in type 2 diabetes and after islet transplantation. Blockade of the RAS could contribute to the development of novel therapeutic strategies in the prevention and treatment of patients with diabetes and in islet transplantation.