Net cation movements were measured in low-density sickle red blood cells (SS RBC) in the presence and absence of oxygen. External Ca2+ (Ca2+o) partially inhibited deoxygenation-induced fluxes of both Na+ and K+. Deoxygenation-induced Na+ influx was reduced by 2 mM Ca2+o to 0.71 +/- 0.04 (SE) of its value in Ca(2+)-free solutions, whereas this ratio was 0.90 +/- 0.05 for K+ efflux (P < 0.01 by paired t-test). Because Ca2+o inhibited Na+ influx more than K+ efflux, net cation loss in deoxygenated SS RBC was higher in the presence of Ca2+o. In separate experiments, Ca2+o reduced deoxygenation-induced Na+ influx to 0.66 +/- 0.03 of its Ca(2+)-free value compared with 0.77 +/- 0.03 for Rb+ influx (P < 0.001), indicating relative selectivity of this effect for Na+ over Rb+. However, this effect is not specific for Ca2+ because other divalent cations also inhibited deoxygenation-induced Na+ and K+ fluxes. Under the conditions of these experiments, no evidence for K+ channel activation was found, indicating that K+ loss measured in deoxygenated SS RBC was mediated by the deoxygenation-induced pathway. These studies show that in the presence of Ca2+o deoxygenation-induced Na+ influx and K+ efflux are unbalanced. This pathway can, therefore, mediate cation loss and contribute directly to cellular dehydration in SS RBC.