Serum gastrin, plasma adrenaline, and blood glucose concentration were measured in 12 patients during insulin-induced hypoglycemia. Six patients had a duodenal ulcer, and 6 patients, 2 to 3 months earlier, had undergone a selective gastric vagotomy due to a duodenal ulcer. The lowest blood glucose concentration was obtained 30 min after the injection of insulin. The rise in plasma adrenaline showed a significant correlation with the degree of hypoglycemia attained. Serum gastrin was approximately doubled in both groups of subjects during the test. There was a strong correlation between rise in serum gastrin expressed in percentage of the basal value and the plasma adrenaline during hypoglycemia. In other experiments, adrenaline was infused intravenously in normal subjects in amounts (6 mug per min for 20 min) resulting in plasma concentrations comparable to those seen during the infusion. It is concluded that adrenaline is a hitherto little recognized factor influencing the gastrin response to hypoglycemia.