The insulin and gastrin response to oral glucose, intravenous glucose, or a protein-rich meal were measured in 44 nondiabetic patients with pernicious anemia (PA) and in 44 control subjects. 36 of the PA-patients had hypergastrinemia, while serum gastrin concentrations in the remaining eight patients were below normal. Three hypergastrinemic PA-patients were in addition studied during an oral glucose loading with synchronous intravenous infusion of gastrin-17. During both oral and intravenous glucose tests blood glucose concentrations were similar in patients and in controls. After ingestion of protein blood glucose concentrations in PA-patients with hypergastrinemia were above those of the controls (P less than 0.05). Parenteral infusion of gastrin-17 during oral glucose loading also increased blood glucose concentrations above the levels observed after glucose alone. In PA-patients with hypergastrinemia the insulin response was augmented in all tests. In patients with hypogastrinemia serum insulin concentrations were lower than normal in the fasting state and during stimulation with glucose intravenously (P less than 0.01). In hypergastrinemic patients serum gastrin concentrations decreased after oral as well as intravenous glucose administration. The decrease was larger during the oral test. In hypogastrinemia oral glucose induced, as in controls, a small initial rise followed by a slow fall in serum gastrin concentrations. No variations were seen in these patients during the intravenous glucose infusion. Gel filtration of serum from hypergastrinemic patients disclosed a decrease in the concentrations of all four main components of gastrin during the glucose loadings. Taken together with earlier studies on the effect of exogenous gastrin the results suggest that endogenous hypergastrinemia induces hyperglycemia and potentiates insulin secretion. In contrast hypogastrinemia is associated with hypoinsulinism.