The purpose of this study was to measure intraerythrocyte and plasma osmolality during graded exercise in humans. Eight volunteers performed a maximal exercise test on a cycle ergometer. Mean corpuscular volume, mean corpuscular hemoglobin concentration, and erythrocyte water content were not significantly (P less than 0.05) affected by the exercise bout in spite of a significant mean increase of 6.7% in plasma osmolality. Interestingly, intraerythrocyte osmolality also increased significantly during the exercise bout, paralleling the response seen in the plasma. In fact, plasma osmolality and intraerythrocyte osmolality demonstrated a significant linear relationship (r = 0.91). These data suggest that during exercise the human erythrocyte has the ability to increase its osmolality in vivo to match that of the plasma. Therefore, this mechanism allows for erythrocyte volume to remain relatively unchanged during exercise despite a significant increase in plasma osmolality.