In seven dogs, prepared with gastrostomies and femoral and portal vein cannulas, insulin hypoglycemia was accompanied by an average of fivefold increase in the rate of acid secretion and the release of gastrin into the portal vein as measured by radioimmunoassay. The average peak of portal vein gastrin was 225 pg. per milliliter (range, 74 to 520). Gastrin release appeared to be triggered abruptly at glucose concentrations of 43 +/- 11 mg. percent. Vagal release of gastrin has been documented by direct measurement of this hormone.