J. PIVARNIK, T. KAYROUZ, and L. C. SENAY, JR. Plasma volume and protein content in progressive exercise: influence of cyclooxygenase inhibitors.Med. Sci. Sports Exerc. Vol. 17, No. 1, pp. 153–157, 1985. Non-steroidal anti-inflammatory compounds such as ibuprofen and naproxen reduce volume loss in sheep following injection of endotoxin. We investigated whether these drugs would change the course of plasma volume reduction during progressive exercise on a cycle ergometer. Subjects [15 males; age (X±SD), 26.3±8.3 yr; height, 180.3±4.5 cm; weight, 76.8±8.0 kg; peak &OV0312;O2, 3.8±0.6 l·min-1] exercised for 5 min at 20, 30, 40, 50, 60, and 70% of peak &OV0312;O2with and without a 24-h pre-treatment with either ibuprofen or naproxen. The two tests for each subject were done a week apart. Blood samples were drawn before and after each exercise level. Based on blood analysis, cyclooxygenase inhibitors significantly altered the retention of protein within the vascular volume during exercise. That is, the degree of concentration of protein per unit of plasma volume loss during exercise was less with drug treatment. In spite of this, ibuprofen and naproxen did not influence fluid shifts during exercise. Use of cyclooxygenase inhibitors probably increased the permeability of vascular endothelium to large molecules, which was only evident when hydrostatic and osmotic events in muscle capillaries caused an increase in bulk flow across the capillary walls.