Introduction: Mechanical ventilation with large tidal volumes causes ventilator induced lung injury (VILI) in animal models. Little direct evidence exists regarding the deformation of airways in vivo during mechanical ventilation, or in the presence of PEEP. The goal of this study was to measure airway strain and to estimate airway wall tension during mechanical ventilation in an intact animal model. Methods: Sprague-Dawley rats were anesthetized and mechanically ventilated with tidal volumes of 6, 12 and 25 cc/kg with and without 10 cm H₂O PEEP. Real-time tantalum bronchograms were obtained at each condition using microfocal x-ray imaging. Images were used to calculate circumferential and longitudinal airway strains, and based upon a simplified mathematical model we estimated airway wall tensions. Results: Circumferential and longitudinal airway strains increased with increasing tidal volume. Levels of mechanical strain were heterogeneous throughout the bronchial tree. Circumferential strains were higher in smaller airways (<800 µm). Airway size did not influence longitudinal strain. When PEEP was applied, wall tensions increased more rapidly than the strain levels suggesting that a "strain limit" had been reached. Airway collapse was not observed under any experimental condition. Conclusion: Mechanical ventilation results in significant airway mechanical strain that is heterogeneously distributed in the uninjured lung. The magnitude of 3 circumferential but not axial strain varies with airway diameter. Airways exhibit a "strain limit" above which an abrupt dramatic rise in wall tension is observed.