The role of pituitary hormones in the ætiology of diabetic ketoacidosis was examined by withdrawing insulin from five pituitary-ablated diabetics for a 12-hour period. The rise in blood glucose and ketone-body concentrations was markedly retarded in these patients when compared with five matched juvenile-type diabetic controls with normal pituitary function. When cortisol replacement in the pituitary-ablated patients was increased to reproduce the high plasma concentrations found in severe ketoacidosis, blood ketones and glucose were increased but were still significantly lower than in the control diabetics. It is concluded that pituitary hormones may be important in the development of diabetic ketoacidosis.