Aldosterone concentration in peripheral plasma (radioimmune assay) and vascular reactivity (by an original tracer method) were determined in 50 patients with essential hypertension of stages I and II and in 25 healthy males. Aldosterone concentration proved to be elevated in half the patients, mostly in those with stage II of the disease. Vascular reactivity in these patients was increased and the two parameters were in good correlation. In the other group the aldosterone level and vascular reactivity were normal and there was no correlation between these two parameters. It is concluded that in some patients with essential hypertension aldosterone plays a pathogenetic role by increasing vascular reactivity.