Hyperactivity of the sympathetic nervous system has repeatedly been assumed to play a role in the pathogenesis of essential hypertension. Particularly in young hypertensive patients, sympathetic nervous activity seems to be slightly elevated. On the other hand an increased pressor effect of noradrenaline has also been reported in some patients with essential hypertension. While neither the slightly elevated sympathetic tone nor the inconstant increased reactivity to noradrenaline could alone be responsible for the blood pressure elevation, both factors together appear to form an important determinant of the level of arterial blood pressure. There is no uniform evidence that sympathetic nervous activity is changed in uncomplicated diabetes. A diminished sympathetic nervous activity in patients with secondary complications is so far not proved. A slight increase in circulatory reactivity to noradrenaline is described independent of the height of blood pressure, however, data in moderate and accelerated hypertensive diabetic patients are not available.