Endothelins (ETs) are potent vasoconstrictive peptides released from the endothelium and other tissues, which act on target cells by receptorial calcium-mediated mechanisms. ET-1 levels are increased in diabetes, and observations suggest the involvement of ETs in the pathogenesis of diabetic angiopathy. However, it is not possible to exclude that ETs might also influence insulin secretion or function. In vivo infusion of ET-1 in rats induces hypoglycaemia and hyperinsulinemia and in vitro incubation with ET-1 stimulates insulin release by mouse islets. Therefore, ETs might be involved in a circulus vitiosus, resulting in hyperinsulinemia and diabetic angiopathy. The purpose of our study was to verify the effect of ET-1 on rat islets, in both the presence and absence of physiological glucose concentration. Moreover, we tested the effect of another isoform of endothelins, ET-3, and verified the involvement of extracellular calcium in such events. Islets were incubated with increasing ET-1 or ET-3, with or without glucose 5.6 mM. Other samples were prepared using calcium-free medium. Incubation in medium containing ET-1 and ET-3, in the presence of glucose and calcium, induced an increase in insulin release. When ET-1 and ET-3 were incubated without glucose and calcium, insulin release was not modified. Our studies demonstrate that: 1) ET-3, like ET-1, stimulates insulin release by rat isolated islets; 2) direct insulin stimulating effect on islets of both ET-1 and ET-3 is evident with physiological glucose concentrations and is calcium mediated. These results support the hypothesis of ET involvement in the regulation of insulin secretion.