Sympathetic activity was determined in 13 ketoacidotic diabetics by evaluation of plasma and urinary catecholamines, before and in the course of medical management. Patients were divided into two groups. Group A (severe ketoacidosis, n = 5) and Group B (moderate ketoacidosis, = 8), depending on plasma glucose, pH and plasma bicarbonate levels. The results showed an enhanced sympathetic activity in all patients before treatment, with significant decrease during therapy. In Group A plasma catecholamines were higher than in Group B, both before and in the course of therapy. A significant correlation was found between basal plasma catecholamines and initial plasma glucose, plasma bicarbonate, hours of therapy and insulin dosage required to obtain plasma glucose levels below 150 mg/100 ml .These results, suggesting a close correlation between glycometabolic control and adrenergic activity, emphasize the role of the sympathetic nervous system as a powerful contrainsular factor in the pathogenesis and metabolic derangement of diabetic ketoacidosis.