Exercise-induced hemoconcentration is a useful mechanism, particularly in heart failure, because it increases oxygen content of blood, perfusing the working muscles; in 50 normal subjects and 50 patients with congestive heart failure, hemoglobin at peak exercise increased by 7 +/- 3% and 5 +/- 3%, respectively. Hemoconcentration was due to fluid flux out of the vascular bed, likely through oncotic forces related to intracellular lactate accumulation and not to red blood cell recruitment from other organs (spleen), because hemoglobin increase, as a percentage, was similar to plasma protein increase.