Seven men and five women triathletes ran 10 km and bicycled 40 km at race pace for the purpose of studying changes in plasma, blood, and red blood cell volumes. A second trial followed 1 wk later with the order of exercise counterbalanced (running first-cycling second; cycling first-running second). Water consumption was measured by providing water bottles on the bike phase, at 3.2 and 6.7 km on the run phase, and at the transition area. Body weight was obtained at the start and end of each phase. A 10-ml blood sample was obtained just before starting and immediately upon finishing each phase. Changes in plasma volume, blood volume, and red cell volume were calculated from hematocrit and hemoglobin values. Changes in blood volume, plasma volume, and red cell volume did not differ between the sexes; therefore, results were combined. Sequential cycling and running caused significant hemoconcentration (-6 to -8% blood volume; -8 to -10% plasma volume) with moderate dehydration (-3 to -4% body weight) despite ad libitum fluid replacement. More severe fluid compartment shifts occurred on the initial phase regardless of exercise mode. Blood volume and plasma volume changes during the second mode of exercise were minor in extent with the major differences occurring in red cell volume. When fluid changes were calculated on a per hour basis, shifts were greater during running than during cycling.