A case of juvenile diabetic ketoacidosis with severe potassium depletion is presented. The patient had electrocardiographic evidence of complete atrioventricular block and prolongation of intraventricular conduction when she was treated. Her diabetic ketoacidosis was due to discontinuance of insulin therapy and substitution of treatment with tolbutamide, resulting in prolonged potassium depletion and leading to flaccid muscle paralysis and respiratory insufficiency so severe that the patient had to be placed in a Drinker respirator. Following her successful diabetic regulation, she had a blatant psychotic reaction and recovered after several days. She was in apparent excellent health when last seen, and her diabetes is well controlled.