The purpose of this study was to investigate the distribution of lactate in plasma and red blood cells (RBC) in capillary blood during and after incremental exercise. We measured capillary plasma lactate and whole blood lactate of 10 subjects during incremental treadmill running and the first 20 min of recovery. To minimize lactate exchange from plasma to RBC between sampling and analysis, a recently developed rapid plasma separation method was used. RBC lactate was calculated. The RBC/plasma lactate concentration ratio decreased from 1.0 (0.85-1.28) before to 0.37 (0.25-0.45) after exhaustive exercise (plasma lactate 15.9 (12.2-19.5)mmol x I(-1), RBC lactate 4.8 (4.0-7.0) mmol x 1(-1)), thus showing that capillary plasma lactate increased much more rapidly than intracellular lactate during incremental exercise. In the first 5 minutes of recovery intracellular lactate still rose while plasma lactate already declined. Then both decreased while the concentration ratio as well as the absolute concentration gradient remained nearly constant (ratio 20 min after exercise termination: 0.43 (0.19-0.54).