The effect on gastric acid secretion of intravenous infusion of insulin was compared with the effect of rapid intravenous injection of insulin in 5 male duodenal ulcer patients and 5 normal men. Intravenous infusion of 0.1 U per kg-hr of insulin produced a peak acid secretory response similar to that produced by rapid intravenous injection of 0.2 U per kg, but greater than that produced by infusion of 0.05 U per kg-hr. Earlier studies showed that rapid intravenous injection of 0.2 U per kg of insulin produced a maximal acid response. During intravenous infusion there was a more gradual decrease in blood sugar and less untoward side effects that after rapid intravenous bolus injection. Since continuous intravenous infusion of 0.1 U per kg-hr of insulin produces maximal acid secretory responses similar to those obtained with rapid intravenous injection of 0.2 U per kg with less untoward effects, it is recommended that intravenous infusion of 0.1 U per kg-hr for 1 1/2 hr be used in those patients in whom insulin testing is indicated.