To investigate the gas exchange response during exercise in horses with mild bronchiolitis (MB), we studied 7 Standardbred trotters. In order to determine pulmonary gas exchange and red cell volume in relation to bodyweight (CV/BW), submaximal graded exercise tests were performed. V̇o2 was monitored from an open bias flow system without valves. Heart rate, respiratory rate and mean pulmonary and systemic artery pressures were measured. Cardiac output and alveolar ventilation were calculated. Arterial and mixed venous blood were drawn for blood gas analysis and pH measurements. Pulmonary gas exchange was assessed by conventional blood gas variables, and the ventilation-perfusion distribution was estimated using the multiple inert gas elimination technique. The dispersion of perfusion and ventilation distribution and the difference between measured arterial oxygen tension and that predicted on the basis of the amount of ventilation-perfusion mismatching and shunt were determined in order to reflect diffusion limitation. The results were compared, using analysis of variance, with data from an earlier study on healthy Standardbred trotters (LH) during a similar exercise test No differences in pulmonary gas exchange were obtained between LH and MB. However, CV/BW, pulmonary artery mean pressure (PAP) and systemic arterial mean pressure (SAP) were significantly higher in MB than in LH. Lactate concentration, arteriovenous oxygen content differences [C(a-v)o2] and pH were significantly lower in MB horses than in LH. We conclude that Standardbred trotters with mild subclinical bronchiolitis can achieve an adequate gas exchange compared to healthy Standardbred trotters, during a graded exercise test; however, the significantly increased CV/BW in MB horses may be a compensatory mechanism for hypoxaemia during maximal work.