One hundred and thirty insulin infusion tests (dose 0.05 u kg-1h-1) were carried out in 87 patients with peptic ulcer or after a vagotomy. During the test ECG tracings were taken and blood was sampled for blood glucose and serum potassium determination. In 31% of the tests electrocardiographic changes were seen. The cardiovascular effects were supraventricular and ventricular ectopic beats, ST-T changes and U waves. In 2 tests there were potentially dangerous arrhythmias. A significant relationship was seen between ECG abnormalities and the age of the patients. Such a relationship was not present between ECG changes and glucose and potassium values. It is concluded that the insulin infusion test is probably safer than the conventional Hollander test, but constant monitoring is still required.