As little is known of the vagal effects on gastric secretion in diabetes mellitus, the relation between graded doses of pentagastrin by vein and gastric acid secretion was measured in 11 men 24 to 52 years old with diabetes mellitus for from 7 to 22 years and 7 controls 22 to 44 years old. After fasting for 12 h the stomach was drained continuously by a suction pump to obtain 15-min pools of gastric juice of which two were used. This procedure was repeated with infusions of from 0.048 to 4.8 mu g pentagastrin/kg hourly. The responses for the 2 groups did not differ significantly although the dose response curve was shifted to the right in the patients. This was interpreted as reflecting a reduced cholinergic response by the gastric glands.