To investigate 1) the blood rheological responses to high training volume and 2) the potential effect of these responses on arterial hypoxemia induced during submaximal running and cycling, 10 triathletes performed an incremental cycle test, 20 minutes of running (R), and 20 minutes of cycling (C). All trials were performed at nearly 75 % of VO2max. Hematocrit (H), blood viscosity (etab), plasma viscosity (etapl), index of erythrocyte rigidity (Tk), changes in plasma volume (DeltaPV), pulmonary diffusing capacity (DLco), and arteriolized blood gas (PaO2) were measured before and after each trial. Pulse oxymetry (SpO2) and cardioventilatory data were collected continuously. A significant increase in etab, etapl, and H was noted after R and C with respect to pre-exercise, whereas DeltaPV decreased, with a greater decrease after C. Tk was significantly higher after R than after C. A significantly greater drop in DLco was noted after C compared with R. SpO2 decreased significantly more during R, as did PaO2. We conclude that blood rheological responses are specific to running and cycling. Cycling induced a sharp decrease in plasma volume, which could partially explain the greater DLco alteration. Running was characterized by an increase in Tk, which could be implicated in the severity of the drop in arterial oxygenation observed.