It is reported that pre-exercise hyperhydration caused arterial O₂ tension of horses performing submaximal exercise to decrease further by 15 mmHg (Equine Vet. J. Suppl. 34: 425-429, 2002). Because hydration status is important to optimal athletic performance and thermoregulation during exercise, the present study examined whether pre-exercise induction of hypervolemia would similarly accentuate the arterial hypoxemia in Thoroughbreds performing short-term high-intensity exercise. Two sets of experiments, namely, control and hypervolemia studies, were carried out on 7 healthy, exercise-trained Thoroughbred horses in random order, 7 days apart. In resting horses, an 18.0 ± 1.8% increase in plasma volume was induced with NaCl (0.30 - 0.45 g/kg dissolved in 1500 ml H₂O) administered via a nasogastric tube, 285 to 290 min pre-exercise. Blood-gas/pH measurements as well as concentrations of plasma protein, hemoglobin and blood lactate were determined at rest and during incremental exercise leading to maximal exertion (14 m/s on a 3.5% uphill grade) that induced pulmonary hemorrhage in all horses in both treatments. In both treatments, significant arterial hypoxemia, desaturation of hemoglobin, hypercapnia, acidosis and hyperthermia developed during maximal exercise, but statistically significant differences between treatments were not found. Thus, pre-exercise 18% expansion of plasma volume failed to significantly affect the development/severity of arterial hypoxemia in Thoroughbreds performing maximal exercise. Although blood lactate concentration and arterial pH were unaffected, hemodilution caused in this manner resulted in a significant (P<0.01) attenuation of the exercise-induced expansion of the arterial to mixed-venous blood O₂ content gradient.