The effects of intravenous infusion of synthetic C-terminal octapeptide of cholecystokinin (OP-CCK) on concentrations of insulin and glucagon in peripheral venous plasma of conscious dogs were studied. Both hormones increased in response to 160 and 480 ng⁄kg⁄h of OP-CCK. The increases to 480 ng⁄kg⁄h were larger than those to 160 ng⁄kg⁄h. Peripheral venous concentrations of glucose and intestinal glucagon-like immunoreactivity were not altered by OP-CCK. OP-CCK, 160 ng⁄kg⁄h, did not enhance insulin and glucagon responses to intravenous infusion of amino acids. The results suggest that insulin- and glucagon-releasing actions of porcine cholecystokinin preparations should not be attributed entirely to gastric inhibitory polypeptide or other impurities contained in these preparations since the synthetic active fragment of cholecystokinin alone increases insulin and glucagon concentrations in peripheral plasma. (Endocrinology97: 549, 1975)