This is an 8 week open-label trial of the antiepileptic agent, levetiracetam, in the treatment of subjects who meet DSM-IV-TR criteria for panic disorder. In addition, prior to and at the conclusion of the open-label trial, subjects will participate in a 1 hour intravenous (IV) lactate infusion to determine whether levetiracetam administration blocks lactate-induced panic attacks.