Eleven panic disorder patients who panicked in response to exogenous cholecystokinin tetrapeptide (CCK-4) were rechallenged after chronic treatment with imipramine. In the rechallenge the patients displayed a marked reduction in the number and intensity of panic symptoms, duration of symptoms, frequency of panic attacks, and cardiovascular responsiveness. This study demonstrates that imipramine can antagonize the panicogenic effects of CCK-4.