In order to evaluate the haemodynamic response to exercise in Standardbred trotters with red cell hypervolaemia (RCHV), 12 trotters with RCHV were compared with 9 normovolaemic (NV) trotters. Haemodynamic data were recorded during exercise at 4 different speeds on a treadmill. Oxygen uptake was determined with an open bias flow system. Pulmonary artery pressure (PAP), systemic artery pressure (SAP), heart rate, packed cell volume (PCV) and plasma lactate and haemoglobin ([Hb]) concentrations were measured. Arteriovenous O2 content difference, cardiac output, stroke volume, pulmonary vascular resistance (PVR) and total systemic resistance (TSR) were calculated. Oxygen uptake, arteriovenous O2 content difference, heart rate, cardiac output, stroke volume, TSR and lactate did not differ between groups. The RCHV horses had significantly higher both mean diastolic and systolic PAP compared to NV horses and this difference increased with higher workload. Further, a higher SAP, PVR, PCV and [Hb] were found in RCHV horses during the course of exercise. Eleven of the RCHV horses, but none of the NV, showed exercise-induced pulmonary haemorrhage on endoscopic examination. The increase in red cell volume, resulting in a high PCV and high total blood volume, is suggested to be an important contributor to both the increased blood pressures in pulmonary and systemic circulation during exercise and to the development of exercise-induced pulmonary haemorrhage in RCHV horses.