1. In thirty-one patients with essential hypertension there was a close relationship between resting diastolic blood pressure and basal plasma noradrenaline levels (r = 0.729, P<O.001).
2. After ganglionic blockade there was a highly significant correlation between change in resting blood pressure and change in plasma noradrenaline (r = 0.813, P<0*001).
3. Patients with the highest resting blood pressure had the largest falls in blood pressure and plasma noradrenaline.
4. These results suggest that the level of the blood pressure in essential hypertension is at least in part due to excess of sympathetic activity.