Plasma noradrenaline, plasma renin and pressor action of exogenous noradrenaline and angiotensin in normotensive subjects and patients with essential hypertension. In normotensive subjects an inverse correlation was observed between the index of sympathetic nervous activity, the plasma noradrenaline concentration during physical exercise, and reactivity to exogenous noradrenaline. The relationship between the index of sympathetic nervous activity and reactivity to noradrenaline was invariably disturbed in age-matched patients with essential hypertension. A multiple regression analysis revealed a highly significant correlation between the combination of both factors and the height of mean arterial blood pressure (r = 0.91). The data suggest that both factors combined, sympathetic nervous activity and pressor response to noradrenaline, are an important determinant of the arterial blood pressure level. An inverse relationship could also be demonstrated between plasma renin concentration and pressor response to angiotensin II in normotensives and hypertensives. However, this relationship was unaltered in the hypertensives. Therefore angiotensin II does not appear to contribute directly to the high blood pressure.