The effects of hypoglycemic sulfonylurea gliclazide on insulin release and its interaction with tumor-promoting phorbol ester in rat pancreatic islets were examined. Lower concentrations of gliclazide stimulated insulin release in the presence of 5.6mM glucose, but the addition of high concentrations of gliclazide resulted in a tendency to reduce insulin release. The stimulatory action of gliclazide on insulin release was abolished in the absence of Ca^<2+>. Gliclazide also failed to stimulate insulin release in the presence of 16.7mM glucose, and 50μM gliclazide inhibited insulin release significantly. A synergistic effect of gliclazide on insulin release was observed in the presence of phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA). Such a combined effect of gliclazide with TPA was inhibited when extracellular Ca^<2+> was not present. These results indicate that gliclazide stimulates insulin release to affect Ca^<2+> handling in the pancreatic B cell.