A B S T R A C T Glucose intolerance has been observed following diphenylhydantoin (DPH) intoxication. Be-cause of this association between DPH and hyperglycemia, the effect of DPH on insulin release in vitro using preparations of isolated islets of Langerhans and pancreatic pieces was examined. In concentrations identical with those considered necessary for adequate anticonvulsant therapy in man, DPH markedly decreases the insulin secretory response of pancreatic pieces to methacholine, 1 ig/ml, tolbutamide, 250 I-g/ml, and glucose, 200 mg/100 ml, without any demonstrable alteration in the oxidative conversion of glucose-1-"C or glucose-6-14C to 14CO2 by isolated islets. This DPH-induced inhibition of insulin secretion is not reversed by higher concentrations of glucose (600 mg/100 ml) or by increasing concentrations of extracellular calcium ion (4-6 mmoles/ liter). 0.1 mm potassium and 10' M ouabain, however, effectively restore the DPH-induced block of insulin secretion in pancreatic pieces. 60 mm potassium ion, on the other hand, not only restores the insulin secretory response to glucose (200 mg/100 ml) but results in an added stimulation of insulin secretion in the presence of DPH. In the presence of DPH, 2'Na accumulation by isolated islets is decreased by 26-40% as compared with controls. Such evidence is considered to indirectly support the postulate that the electrophysiological properties of DPH on the pancreas are due to a stimulation of the membrane sodium-potassium-magnesium ATPase. INTRODUCTIONIn 1965 an association between hyperglycemia and diphenylhydantoin (DPH) was described in rabbits (1). This observation was soon followed by the clinical recognition of hyperglycemia and nonketotic hyperosmolar