A technique for studying the regulation of insulin secretion using the in situ perfused pancreas of the European silver eel (Anguilla anguilla) has been developed. The organ preparation (pancreas + intestine) was perfused via the intestinal artery at a flow rate of 1 ml/min and the venous effluent collected from the cannulated hepatic portal vein. No significant variations of flow rate, perfusion pressure, perfusate pH, or total tissue oxygen consumption occurred over a period of 110 min. The viability of the preparation was evaluated by studying the insulin secretory response to glucose and to arginine during a 60-min stimulatory period. Elevation of perfusate glucose concentration to 30 mM or perfusion with arginine (10 or 20 mM in the presence of 2.7 mM glucose), resulted in a biphasic pattern of insulin release. However, the initial peak response to arginine was more rapid and of significantly greater magnitude at both concentrations than the response following glucose. This, together with the significantly greater amount of insulin secreted in response to the amino acid, indicated that in eels, arginine is a more potent insulin secretagogue than glucose. © 1979.