A 2d case of diabetes mellitus developing in a patient with well-established Addison''s disease was described. The onset of diabetes mellitus in this patient with previously well-controlled Addison''s disease was characterized by anorexia, loss of weight, lassitude, fall in blood pressure and increased requirement of desoxycorticosterone acetate. Institution of insulin therapy at a time when large doses of DCA and NaCl were being administered precipitated the development of "low K" muscular weakness. Regulation of diabetes with small doses of insulin was followed by striking improvement in appetite, desire for fatty foods, increased wt., restoration of blood pressure and general sense of well-being. Marked improvement in personality was observed in conjunction with the persistent hyperglycemia. No change in the abnormal electro-encephalogram was noted. Withdrawal of insulin waa followed by an increased blood-sugar level, and an increased excretion of N, P and K. With adequate DCA, which prevented excessive dehydration, no return of ketonuria was observed during the aggravation of diabetes induced by insulin withdrawal. The marked insulin senstivity of this patient suggests that protamine zinc insulin adm. might prove to be extremely hazardous. The adm. of a single dose of 33 mg. of 11-dehydro-17-hydroxycorticosterone was followed by an elevation in blood-sugar level, a lowering of non-protein R.Q., increased excretion of glucose, N and P accompanied by the presence of ketonuria and increased excretion of Na and Cl. It appears from studies on this patient that the inability of patients with uncomplicated Addison''s disease to maintain an adequate fasting blood-sugar level is not dependent solely upon deficiency of the adrenal cortical hormone.