We studied 14 outpatients with panic disorder (8 women and 6 men; mean ± SD age, 37 ± 10 years) who were moderately ill judging by their mean ± SD total prescan Panic Disorder Severity Scale (PDSS)%20 score (13 ± 4; n = 13). The PDSS samples 7 symptom domains (each scored on a scale from 0-4) relevant to panic disorder, including frequency of panic symptoms, distress during panics, phobic symptoms, anticipatory anxiety, and functioning (see Shear et al%20 for a review of psychometric properties). All patients had a weekly panic attack frequency of 1 or more in the month before study entry. Baseline mean ± SD scores were as follows: Hamilton Anxiety Rating Scale (HAM-A),%21 17 ± 8 (n = 14); 25-item Hamilton Depression Rating Scale (HAM-D),%22 20 ± 10 (n = 14); 17-item HAM-D,%23 14 ± 6; and Clinician-Rated Anxiety Scale (CRAS) (contains 37 items, each rated on a scale from 0-4, covering panic attacks, phobias, and many symptoms of generalized anxiety),%24,25 31 ± 16 (n = 14). All patients had normal physical examination findings and normal results on follow-up tests, including urine toxicology, urinalysis, electrocardiogram, serum electrolytes and glucose, liver and thyroid function tests, blood cell count and serum gonadotrophin levels (for women), and human immunodeficiency virus testing. Of the women, 1 was menopausal, 1 was perimenopausal, 3 were at the end of their menstrual cycle just before the scan, 1 was midcycle, 1 was in the first half of the cycle, and 1 was in the second half of the cycle. Patients met DSM-IV criteria%26 for a current principal diagnosis of panic disorder with or without agoraphobia. The panic diagnosis was confirmed using a semistructured interview (either the Anxiety Disorders Interview Schedule DSM-IV version%27 or the Structured Clinical Interview for DSM-IV%28) administered by experienced research personnel under the supervision of the principal investigator (A.W.G.).