Intravenous injection of insulin increased heart rate approximately 20% in six alloxan-diabetic rabbits. Blood glucose concentrations after insulin did not decrease below the fasting level of non-diabetic animals and none of the rabbits had signs of hypoglycermia. Intravenous injection of saline or insulin solvent had no effect on heart rate. The stimulatory effect of insulin on heart rate was not influenced by autonomic nervous blockade by propranolol or by propranolol plus atropine.