We examined adrenergic regulation in patients with panic disorder by challenging 10 patients and 14 age-matched and sex-matched controls with intravenous infusions of clonidine hydrochloride (2 micrograms/kg), an alpha 2-adrenoreceptor agonist. Growth hormone, 3-methoxy-4-hydroxyphenylglycol (MHPG), blood pressure, heart rate, and behavioral (anxiety, sedation) responses were monitored. The data replicated the previously reported finding of blunted growth hormone (GH) responses to clonidine in patients with panic disorder. Reported abnormalities in MHPG, cardiovascular, and behavioral responses of panic patients to clonidine infusion were not replicated. The robustly blunted GH response to clonidine in panic patients supports the adrenergic dysregulation hypothesis of panic disorder, but alternative interpretations of this finding are available and further study is needed.