1. A stepwise infusion of phentolamine was administered to 26 patients with mild uncomplicated essential hypertension, whose basal renin was inversely related to age.
2. Phentolamine caused a dose-related increase of plasma renin activity, which was independent of mean blood pressure and heart rate changes.
3. The maximum increment of plasma renin activity was inversely related to age and the dose-response curve of plasma renin activity was steeper in the younger group than in older patients.
4. Our data do not support the hypothesis than an increased α-adrenergic tone in elderly people may explain the age-dependent decline of renin in uncomplicated essential hypertension.