The influence of clonidine pretreatment on psychopathological, endocrine and respiratory effects of cholecystokinin tetrapeptide (CCK-4) was characterized. Patients with panic disorder (DSM-III-R) were given 50 micrograms CCK-4 i.v. at 1100 hours on 2 separate study days. In a randomized double-blind design they were additionally infused with 150 micrograms clonidine or placebo from 1040 to 1110 hours. After CCK-4 all patients experienced symptom attacks. No effects of clonidine on panic psychopathology or blood gas parameters were observed. After CCK-4, in the clonidine condition the pituitary release of adrenocorticotropin (ACTH) and prolactin was seemingly enhanced compared to placebo. Our results suggest that CCK-4-induced panic attacks are not suppressible by presynaptic alpha-2 receptor stimulation. Moreover, they point to a synergistic postsynaptic action of clonidine to CCK-4 upon pituitary hormone secretion. The diverging sites of action might possibly explain the discrepancies of psychopathological alterations and stress hormone secretion.