In 55 children with congenital heart defects (aged 6 months and older), hematocrit, hemoglobin concentration, acid-base status, oxygen pressure, oxygen saturation, and 2,3-DPG content of erythrocytes were determined in blood taken from the vena cava superior. The data were correlated with each other as well as with parameters obtained during cardiac catheterization. Whereas an increased hematocrit was seen only at a difference in arteriovenous oxygen saturation of more than 30%, or a central venous saturation below 65%, a close correlation between 2,3-DPG concentration in erythrocytes and these parameters was obtained already in the normal range (r = 0.79, and r = 0.78, respectively). A relatively close correlation was also found between 2,3-DPG concentration and half-maximal oxygen saturation pressure (T50 value; r equals 0.73), and the T50 values and the central venous oxygen saturation (r equals 0.61), respectively. Because of the shift to the right of the O2-binding curve the functional oxygen transport capacity was increased by an average 15% in children with acyanotic heart defects (mean T50 value of 31.6 mm Hg), and by 40% in children with cyanotic heart defects (mean T50 value of 34.6 mm Hg), as compared to healthy children (mean T50 value of 28.1 mm Hg). A marked influence on the position of the O2-binding curve and hence on oxygen release of the blood in the periphery is therefore exerted by the 2,3-DPG concentration, whose effect was about twice that of the Bohr effect in the pH range measured.