We recently reported that the Cl(-)-dependent K+ (K:Cl) efflux, which can be stimulated by cell swelling in the presence of inhibitors of the Na+ pump (ouabain) and of the Na-K-Cl cotransport (bumetanide), is highly active in young AA and SS RBCs. We report here that deoxygenation inhibits volume-stimulated K:Cl efflux in SS and reticulocyte-enriched density-separated SS and AA RBCs. In SS whole blood, the K:Cl efflux stimulated by hypotonic (220 mOsm) swelling was reduced from 9.2 +/- 2 (mean +/- SE) in oxygen to 2.7 +/- 1.9 (mmol/L cell/h = flux units = FU) (n = 4) under deoxygenated conditions (P less than .005). Deoxygenation also decreased the acid pH-stimulated K:Cl efflux from 5.9 +/- 1.5 to 3.7 +/- 1.1 FU (n = 3) (P less than .025) but did not inhibit NEM-stimulated K:Cl transport. The effect of deoxygenation on density-separated SS cells is similar: When fraction SS2 (reversible discocytes) is deoxygenated under hypotonic conditions, the K:Cl efflux is reduced by 50%. In reticulocyte-enriched AA cells obtained from anemic patients, deoxygenation under hypotonic conditions also reduces K+ efflux by 50%. In SS cells only, deoxygenation under isotonic conditions results in an increased Cl(-)-independent K+ efflux. Because ionized Mg2+ in the cytosol increases during deoxygenation, we investigated the effect of external and internal Mg2+ on the volume-stimulated K:Cl efflux. Removal of external Mg2+ did not influence the rate of transport in oxygenated cells. When internal Mg2+ was clamped at 0.15 mmol/L with A23187 and EDTA at ionized cytosolic Ca2+ = O, however, the inhibitory effect of deoxygenation on the K:Cl efflux was eliminated. We conclude that deoxygenation inhibits the volume-stimulated, Cl(-)-dependent K+ efflux in AA and SS young red cells by concomitantly increasing ionized cytosolic Mg2+.