To examine the role of natural conditioning mechanism in the attacks of bronchial asthma, some clinical and laboratory examinations were carried out. The results obtained were as follows ; l. In a series of 85 asthmatic patients, the occurrence of natural conditioning mechanism in the precipitation of attacks was demonstrated in 72.3%. The patients' group which showed the appearance of high degree of natural conditioning was characterized by (1) higher percentage of females (71.4%), (2) Ionger duration of disease state (average 11.8 years), (3) higher percentage of perenial type (51.0%) and (4) higher percentage of severe cases (45.7%).2. The aggravation or improvement of respiratory function could be conditioned in the patients of bronchial asthma by using acetylcholine or adrenalin as an unconditioned stimulus. 3. The conditioned reflex asthma could be elaborated in guinea pigs by using histamin inhalation as an unconditioned stimulus. The significant increase of noradrenaline as the effect of conditioned stimulus was demonstrated in the organs of the guinea pigs in which respiratory conditioning was elaborated., This result shows that the respiratory conditioning was accompanied by simultaneous acute activation of sympathetic-adrenomedullary system, as the result of conditioned emotional reaction of anxiety or fear. 4. In view of the significant role played by the natural conditioning mechanism in the clinical course of bronchial asthma, a new therapeutic approach of behavior therapy would become an important tool in the treatment of asthmatic patients, especially in that of obstinate or chronic cases.