The changes in the red cell volume and the plasma chloride level were measured when the blood CO₂ content was altered by equilibration with the atmospheric air or pure CO₂ for 20 minutes. The red cell volume was expressed in terms of hematocrit and mean corpuscular volume (M.C.V.). The results obtained were as follows. 1) On equilibration with the atmospheric air, the MCV and the plasma chloride level were 91.6±1.26 c.μ and 110.7±6.28mEq/L. respectively. 2) On equilibration with pure CO₂, the MCV and the plasma chloride level were 109.6±2.0 c.μ and 90.7±5.17 mEq/L. respectively. 3) When the blood was subjected to equilibration with the atmospheric for air 20 minutes after equilibration with pure CO₂ for the same period of time the MCV and the plasma chloride level were 89.9±6.34 c.μ and 100.3±5.50 mEq/L. respectively. From the above results it can be concluded that an increase of the blood CO₂ content in the experimental condition causes definitely a decrease of the plasma chloride level and a concomitant increase of the red cell volume, and that a decrease of the blood content CO₂ in the experimental condition causes definitely an increase of the plasma chloride level and a concomitant decrease of the red cell volme. Apparantly there exists a parallel relationship between the extent of the decrease of the plasma chloride level and that of the increase of the red cell volume when the blood CO₂ content increased in the experimental condition. When the blood CO₂ content decreased, the extent of the decrease of the red cell volume exceeds that of the increase of the plasma chloride level.