Oxygen dissociation curves (ODC) in whole blood and organic phosphate concentrations in red cells were determined in 10 highly trained male athletes (TR), 6 semitrained subjects (ST) who played sports regularly at low intensities and 8 untrained people (UT). In all groups standard ODCs (37 degrees C, pH 7.40, PCO2 approximately 43 Torr) at rest and after a short exhaustive exercise were nearly identical, but PO2 values measured immediately after blood sampling and corrected to standard conditions tended to fall to the right of the in vitro ODC. Elevated P50 in the physically active [28.65 +/- 1.4 Torr (3.81 +/- 0.18 kPa) in ST, 28.0 +/- 1.1 Torr (3.73 +/- 0.15 kPa) in TR, but 26.5 +/- 1.1 Torr (3.53 +/- 0.15 kPa) in UT] were partly caused by different [DPG] (11.9 +/- 1.3 mumol/GHb in UT, 13.3 +/- 1.5 mumol/GHb in TR, 13.8 +/- 2.2 mumol/gHb in ST). There were remarkable differences in the shape of the curves between the groups. The slope "n" in the Hill plot amounted to 2.65 +/- 0.12 in UT, 2.74 +/- in ST and 2.90 +/- 0.11 in the TR (2 p against UT less than 0.001), leading to an elevated oxygen pressure of about 2 Torr (0.27 kPa) at 20% saturation and an augmented oxygen extraction of 5--7 SO2 at a PO2 of about 15 Torr (2kPa), which might be favorable at high workloads. The reason for the phenomenon could be an increased amount of young red cells in the blood of TR, caused by exercise induced hemolysis.