Fourteen studies of gastric acid secretion in a basal hour and in the two hours after a single intravenous injection of soluble insulin (0.01 to 0.40 units/kg) were performed in a healthy man. The peak acid output after insulin (measured as the two consecutive 15-minute samples giving the highest acid output) was significantly correlated with the lowest concentration of blood glucose, the fall in blood glucose, the rate of fall of blood glucose, and the maximum fall of blood glucose in any 15 minutes. Peak acid outputs after insulin were similar over the range 0.1 to 0.2 units/kg, and greater than at lower or higher doses. These results are contrary to the accepted assumption that insulin-stimulated acid secretion is an ;all-or-none' phenomenon. They support instead the hypothesis that insulin hypoglycaemia provides a quantitative glycopenic stimulus producing quantitative vagal acid response. Extreme hypoglycaemia, below about 15 mg/100 ml of blood glucose, inhibits insulin-stimulated acid secretion.