The mechanical stress imposed on the lungs during breathing is an important modulator of airway responsiveness in vivo. Our recent study demonstrated that continuous positive airway pressure (CPAP) applied to the lungs of non-anesthetized, tracheotomized rabbits for four days decreased lower respiratory system responsiveness to challenge with ACh. In addition, airway segments excised from the lungs of these animals and studied in vitro exhibited reduced contractility. However, the mechanism for this reduction in contractility was not determined. The stress-induced decrease in airway responsiveness could have resulted from alterations in the excitation-contraction coupling mechanisms of the smooth muscle cells, or it might reflect changes in the structure and/or composition of the airway wall tissues. In the present study, we assessed the effect of prolonged chronic stress of the lungs in vivo on airway smooth muscle force generation, MLC-phosphorylation and airway wall structure. To enhance the potential development of stress-induced structural changes, we applied mechanical stress for a prolonged period of time of 2 - 3 weeks. Our results demonstrate a direct connection between the decreased airway responsiveness caused by chronic mechanical stress of the lungs in vivo and a persistent decrease in contractile protein activation in the airway smooth muscle isolated from those lungs. The chronic stress also caused an increase in airway size, but no detectable changes in the composition of the airway wall.