Hemodynamic and pharmacological characteristics of 14 patients with clinical findings suggesting hyperdynamic β-adrenergic circulatory state (including 12 with hypertension of unknown cause) were compared with 13 essential hypertensive patients and 25 normotensive volunteer subjects. Patients with cardiac symptoms had significantly higher resting cardiac index and heart rate than normotensive or asymptomatic hypertensive individuals. Graded isoproterenol hydrochloride infusion increased heart rate significantly more in the symptomatic group than in the others, indicating increased β-receptor hyperresponsiveness. In nine of the 14 patients, isoproterenol produced an hysterical outburst, almost uncontrollable, which was promptly reversed with propranolol hydrochloride but not placebo. It therefore seems reasonable to ascribe the syndrome of cardiac awareness, increased heart rate responsiveness to various stimuli, and hyperkinetic circulation to increased β-adrenergic receptor reactivity. This conclusion is supported by remission of symptoms and reduction of arterial pressure with propranolol in all treated patients.