Five healthy males volunteered to exercise for 20 min on a motor-driven treadmill at five submaximal intensities (30, 45, 60, 75, and 90% of VO2 max). Peripheral venous blood samples were drawn from an indwelling catheter prior to and at 9, 14, and 19 min of each exercise bout. Blood samples were assayed for whole-blood hemoglobin, total plasma protein concentrations, and hematocrit, with plasma water concentration calculated from these values. The plasma concentration of the electrolytes sodium (Na+), potassium (K+), total calcium (Catot), ionized calcium (Ca2+), chloride (Cl-), and inorganic phosphorus (Pi) were also determined. With plasma and blood volumes the total plasma contents of each of the measured constituents and the concentration of each electrolyte per liter of water were calculated. Statistically significant linear increases in plasma concentrations of Na+, K+, and Cl- relative to exercise intensity were observed, with linear decreases in plasma contents of Na+ and Cl- and linear increases in K+ content. Plasma Pi concentration decreased with a Pi increased content, with plasma Catot concentration elevated at the highest two work loads. Plasma Catot content increased linearly with exercise intensity and duration. Plasma water concentration and content decreased with exercise intensity, resulting in no change in electrolyte concentration per liter of water except at the highest two exercise intensities. These data suggest that extrapolation from plasma electrolyte concentration to muscle concentration and/or content is not supported. Changes in plasma volume and plasma water must be considered when postulating a role for electrolytes in the physiological responses of the human to exercise.