The isolated perfused rat pancreas was used to investigate the effect of oleic acid on glucagon secretion in response to 10 mmol/l arginine. In the absence of oleic acid and at 2.5 mmol/l calcium, arginine induced a biphasic glucagon secretion. At lower extracellular calcium concentration (1.0 mmol/l), the second phase of glucagon release was reduced, the first phase being unchanged. In the presence of 1,500 mumol/l oleic acid, the glucagon response to arginine was also biphasic, but second phase release was markedly inhibited, the first phase glucagon release being unchanged. Such an effect was not obtained when oleic acid concentration in the medium was 750 mumol/l. These results demonstrate that high concentrations of oleic acid inhibit glucagon secretion in response to arginine from the isolated perfused rat pancreas and support the concept that circulating free fatty acid levels are involved in the control of glucagon secretion.