Equine recurrent airway obstruction (RAO) is associated with airway inflammation and bronchoconstriction in clinically affected horses. Horses demonstrating severe pulmonary compromise develop concurrent secondary pulmonary hypertension. The development of pulmonary hypertension is well documented in RAO affected horses, however, it is not known how rapidly increases in pulmonary artery pressure occur after the onset of RAO. It is also not known if pulmonary hypertension resolves concurrently with resolution of RAO. The goal of this study was to measure pulmonary artery pressure in RAO affected horses during the development and resolution of RAO. To accomplish this, three RAO affected and three normal horses were placed in a challenge environment where clinical parameters, pulmonary function, right heart and pulmonary artery pressures were measured on day 1, 3 and 5. After evaluating horses on day five, their environment was modified to reduce exposure to respirable debris and anti-inflammatory medication (dexamethasone) was initiated. Identical clinical parameters were measured on days 7 and 9. In our study, the arterial oxygen content in RAO horses was significantly less that that of control horses from day 1 through day 9. A concurrent increase in pulmonary artery pressure also developed on day 3 in RAO affected horses, and persisted through day 5. While some trend towards a difference between groups was noted, no other significant differences were observed between RAO and normal horses. These findings suggest that horses with severe RAO also develop significant increase in pulmonary artery pressure, which rapidly resolves with appropriate management of RAO. / Master of Science