In 21 unoperated male duodenal ulcer patients, i.v. 2-deoxy-D-glucose (2 DG) produced significantly higher gastric acid responses than i.v. insulin. The acid response appeared earlier with 2 DG than insulin. The dose of 2 DG (50 mg/kg) was probably submaximal and that of insulin (0.2 U/kg) is regarded as maximal for acid secretion in duodenal ulcer patients. The side effects with 2 DG and insulin were similar, appeared in about equal frequency and required no treatment. In all patients, 2 DG increased the blood glucose level. The potential value of 2 DG is for testing the completeness of vagotomy, but the experience with 2 DG in man is still too limited to warrant its use in routine clinical secretory tests.