Screening (Visit 1): Subjects will provide written informed consent after an explanation of all study procedures. The Structured Interview for DSM-IV anxiety disorder module will be administered to confirm the diagnosis of panic disorder. A complete psychiatric and medical history will be obtained and a physical examination, including vital signs, will be performed. Subjects must experience a minimum of one panic attack per week over the month preceding the study. Inclusion/exclusion criteria will be reviewed and subjects will be assessed for compliance with these criteria. Subjects will be given a panic attack diary to record the number of panic attacks they experience each day. Subjects will be given the API. All female subjects who are able to have children will have a blood sample drawn for laboratory evaluation of β HCG. Subjects will be asked to return in one week.