Alterations of red cell pH (pHc) and distribution ratios of H+ (gammaH+) and Cl (gammaCl-) between plasma and red blood cells with oxygenation of blood were studied in human blood (audult and fetal) and ruminant blood (bovine, goat and sheep). The experiments were carried out at a plasma pH of 7.0 to 7.7 and at 37 degrees C. In human blood pHc of fully oxygenated blood was 0.035 pH lower than that of fully deoxygenated blood in all ranges of plasma pH studied. In ruminant blood, however, the differences in pHc between oxygenated and deoxygenated blood were 0.011 in ox, and 0.003 in goat and sheep, all of them not being significant. The decreases in gammaH+ accompanying oxygenation were in fairly good agreement with those in gammaCl- in human blood and amounted to about 0.05. In ruminant blood, in spite of virtually zero changes in gammaH+ with oxygenation, the decreases in gammaCl- were slightly greater than those in human blood. There might be a species difference in the mechanisms of distribution of Cl- and H+ across the red cell membrane.