The simultaneous Henderson-Hasselbalch equations in plasma and red cell were solved in order to obtain the C02 dissociation curve of oxygenated blood. In order to solve the above two equations the following equation was added, in which the relationship between the intracellular (4pH) cand the extracellular pH change (dpHP) was defined as follows:pHc=(1+a)dpHP, where 1 +a is a factor to be determined from experimental data on Donnan's ratio for H+. From the solution, the ratio of bicarbonate shift to the C02 quantity released out of or combined with hemoglobin was calculated. The solution was validated by comparing the above ratio between the theoretical and experimental data. The C02 contents calculated at 12 Torr in whole blood, red cell, and plasma compartments show good agreement with the respective analyzed values. When the buffer values of hemoglobin and plasma buffer protein were 70.0 and 7.5 mmol/ (liter plasma • pH), respectively, a=-0.21 +0.05 • dpHP, and the Donnan's ratio for HC03-was assumed to be 0.7 at pH=7.33, the theoretical C02 dissociation curve fitted well with the experimental curve. The C02 dissociation curve of deoxygenated blood was expressed by adding the measured Haldane effect to the C02 content of oxygenated blood. This additive characteristic in turn made it possible to estimate carbamate contribution in the Haldane effect. In a previous paper (MocuIzuKI et al., 1982) it was clarified that the C02 dissociation curve could be expressed by an exponential function of PC02, whose exponent and multiplying factor had a consistent relation to each other. Therefore, a certain relation such as the Henderson-Hasselbalch equation was