The relation between dose of insulin and gastric secretion of acid and pepsin was studied in 5 healthy adult men. Doses of insulin ranged from 0.025 to 0.4 unit per kg intravenously. The peak 15-min acid output increased with increasing insulin dose up to a maximum observed response with 0.2 unit per kg; 0.4 unit per kg did not produce a higher peak response. The peak response to 0.2 unit per kg (8.4 mEq per 15 min) was equivalent to 89% of the peak response to a maximal dose of betazole. Peak pepsin output continued to increase up to the highest dose used (0.4 unit per kg). Before a standard dose for clinical testing for incompleteness of vagotomy can be established, further tests must be done to determine whether the dose required for peak response is the same in partially vagotomized as in intact subjects.