Increased beta-adrenergic receptor sensitivity could account for many aspects of panic disorder. We tested this hypothesis by comparing 14 patients with six normal control subjects. The controls and eight patients had 14 blood samples taken, and heart rate and BP measured, during a four-hour protocol that included supine rest, a posture and isometric exercise stimulus, and a series of up to seven logarithmically increasing bolus intravenous doses of isoproterenol hydrochloride. The other six patients were studied only at rest. Patients had markedly elevated resting heart rate, substantially elevated levels of plasma epinephrine, cortisol, and growth hormone, mildly elevated plasma norepinephrine levels, and decreased heart rate responses to isoproterenol. These results suggest that beta-adrenergic receptor response is not increased, and may be decreased, in patients with panic disorder. Receptor down-regulation could result from the increased adrenergic function that these patients demonstrate, even in the absence of panic attacks.