Rationale: In human athletes, airway inflammatory cells were found to be increased in induced sputum or bronchial biopsies. Most data were obtained after exposure to cold and dry air at rest or during exercise. Whether training affects epithelial and inflammatory cells in small airways is unknown. Objectives: To test whether endurance training under standard environmental conditions causes epithelial damage and inflammation in the small airways of mice. Methods and Measurements: Formalin-fixed, paraffin embedded lung sections were obtained in sedentary (total n=14) and endurance-trained (total n=16) Swiss mice at baseline and after 15, 30 and 45 days of training. The following variables were assessed (morphometry and immunohistochemistry) in small airways (basement membrane length <1 mm): a) integrity, proliferation and apoptosis of bronchiolar epithelium; b) infiltration, activation, and apoptosis of inflammatory cells. Main Results: Compared to sedentary mice, bronchiolar epithelium of trained mice showed progressive loss of ciliated cells, slightly increased thickness, unchanged goblet cell number and appearance, increased apoptosis and proliferation (PCNA) (p<0.001 for all variables). Leukocytes (CD45⁺ cells) infiltrated airway walls (p<0.0001) and accumulated within the lumen (p<0.001); however, apoptosis of CD45⁺ cells did not differ between trained and sedentary mice. NF-B nuclear translocation and IB phosphorylation were not increased in trained compared to sedentary mice. Conclusions: Bronchiolar epithelium showed damage and repair associated with endurance training. Training increased inflammatory cells in small airways but inflammatory activation was not increased. These changes may represent an adaptive response to increased ventilation during exercise.