Horses with recurrent airway obstruction (RAO) present many similarities with human asthmatics including airway inflammation, hyperresponsiveness, reversible obstruction, and increased NF-kappaB expression. Studies in experimental asthma models have shown that transcriptions factors such as activator protein-1 (AP-1), GATA-3, cyclic AMP response element binding protein (CREB) and CAAT/enhancer binding protein (C/EBP) may also play an important role in airway inflammation. The purpose of this study was to measure DNA binding activity of these transcription factors in the airways of horses with RAO and to compare it to pulmonary function and bronchoalveolar lavage fluid (BALF) cytology. Seven horses with RAO and six control animals were studied during a moldy hay challenge and after 2 months at pasture. Pulmonary function, BALF cytology and transcription factors' activities in bronchial brushings were measured during hay and pasture exposures. During moldy hay challenge, RAO-affected horses developed severe airway obstruction and inflammation and a significantly higher airway AP-1 binding activity than in controls. After 2 months on pasture, pulmonary function and airway AP-1 binding activity were not different between RAO and control horses. The DNA binding activity of CREB in airways of RAO-affected horses increased significantly after 2 months at pasture and became higher than in controls. A significant positive correlation was detected between AP-1 binding activity and indicators of airway obstruction and inflammation. Airway GATA-3, CEBP and CREB binding activities were negatively correlated with indices of airway obstruction. However, contrarily to CREB binding activity, GATA-3 and CEBP binding activities were not different between RAO and control horses and were unaffected by changes in environment. These data support the view that AP-1 and CREB play a role in modulating airway inflammation in horses with RAO.