Stress is one of the important factors influencing bronchial asthma, but many questions still remain unanswered. To clarify this point we examined airway hypersensitivity before and after electric shock stress and the role of substance P in an animal model of asthma. We determined airway hypersensitivity to histamine and the substance P levels in serum, bronchoalveolar lavage fluid, and bronchial tissue before and after electric shock stress in biphasic asthma-responsive guinea pigs which had been sensitized using ovalbumin. The cell components in bronchoalveolar lavage fluid were also examined. Airway hypersensitivity to histamine (4.9-156 micrograms/ml) was significantly increased (p < 0.01) by electric shock stress. The substance P level was also significantly increased in plasma and bronchoalveolar lavage fluid, but it was significantly decreased in bronchial tissue. The number of eosinophils in bronchoalveolar lavage fluid increased significantly after electric shock stress. These findings demonstrated that airway hypersensitivity to histamine was increased by stress and suggested that substance P, as well as eosinophils, contribute to the pathogenesis of hypersensitivity.