Effect of zinc ion on the glucose-induced insulin and arginine-induced glucagon release from the isolated perfused rat pancreas was studied. Zinc inhibited insulin release dose-dependently at the concentration of 100 and 200 μM. This inhibition by zinc was observed only in the second phase of glucose-induced insulin release, but not in the first phase. The inhibition by zinc was partial and reversible. Zinc, at 200 μM, also inhibited arginineinduced glucagon release reversibly. Zinc inhibited the activity of phosphodiesterase from rat brain activated in the presence of calcium ion and calmodulin in a dose-dependent manner similarly to trifluoperazine and N-(6-aminohexyl)-5-chloro-1-nephthalenesulfonamide (W-7), specific calmodulin inhibitors. These results suggest that the inhibition of insulin or glucagon release by zinc ion is at least due to the interaction of zinc with the phosphodiesterase activating system, calcium-calmodulin in the pancreatic islets of Langerhans.