Insulin and pancreatic somatostatin secretions were studied after stimulation with an arginine infusion (5 mmol/l) in isolated perfused pancreata of adult streptozotocin-diabetic rats. In the presence of 2.8 mmol/l glucose, arginine clearly stimulated insulin and somatostatin secretions in diabetic rats, whereas it was ineffective in normal rats. Thus, not only the B-cells, but also the D-cells of the pancreas from streptozotocin-diabetic rats are hypersensitive to arginine. The infusion of insulin (4 U/l) did not modify this hypersensitivity of the D-cells to arginine in pancreata of streptozotocin-diabetic rats.