The plasma insulin response to an oral glucose load was studied in 14 patients with reactive hypoglycemia-based on blood glucose values of 45 mg/100 ml or below following 100 g of glucose. In reactive hypoglycemia, no single mechanism involving insulin secretion could explain the development of low blood glucose values. Six patients with reactive hypoglycemia demonstrated elevated plasma insulin values in association with normal blood glucose levels. In 5 patients the results of glucose tolerance tests suggested mild diabetes mellitus, and plasma insulin levels were increased. Three patients with reactive hypoglycemia had essentially normal glucose tolerance tests and no abnormality in plasma insulin response was noted.