Two patients with pheochromocytoma predominantly secreting noradrenaline, were studied. Intravenous glucose tolerance tests were performed: in the preoperatory period, under alpha adrenergic blockade with pentolamine and one month after the tumor was removed. Serum glucose and insulin concentrations were determined. Proinsulin was separated from the insulin in Sephadex G-50 fine columns. In the preoperatory study, insulin secretion was less during the first 10 minutes after glucose administration, when comparing to that observed in two other studies. During pentolamine blockade, insulin secretion was at least three times higher than the preoperatory test; however, hypoglycemia was not observed. Proinsulin levels were similar to the observed in a normal individual in the pre and postoperatory test during pentolamine blockade, and were found significantly elevated. It is postulated that hypoglycemia was not produced during the late test because more than 50 per cent of immunoreactivity measured as insulin, corresponded to proinsulin whose biological activity is 10 times less than that of insulin.