15 pantients who met the DSM-III criteria of panic disorder and 15 age- and sex- matched normal control subjects underwent the lactate infusion tests. Before the test, all of the patients were evaluated by Hamilton scale for anxiety and during the test, the intensity of the lactate- induced physical and psychological symptoms comparing to the severest level of their usual panic attacks were measured respectively using 5-point scale developed by authors. The lactate infusion tests were conducted under the single- blind setting using 0.5 mole sodium lactate solution. The results were as follows; 1 ) 80% of the patients showed panic attacks during the lactate infusion tests but none of the controls showed panic attack. 2) The lactate-induced physical symptoms were rated significanty higher than the lactate-induced psychological symptoms. There were significant correlation between the physical sympt-oms and the psychological symptoms. 3) Hamilton scale for anxiety correlated signifcantly with the lactate- induced psychological symptoms but not with the physical symptoms. Our findings imply that the panic disorder is a biological disease in which the artiHcial symp-tom provocation is possible by lactate infusion ane the lactate-induced physical symptoms are the core and pathognomonic symptoms of panic disorder. The psychological symptoms of panic disorder arc the secondary phenomena which may be influenced by the patients cognitive functions and previous learned experiences. Therfore, the psychological symptoms should not influence the diagnosis of panic disorder.