The normal red blood cell (RBC) membrane is remarkable for its durability (eg, preservation of permeability barrier function) despite its need to remain deformable for the benefit of microvascular blood flow. Yet, it may be hypothesized that the membrane's tolerance of deformation might be compromised under certain pathologic conditions. We studied this by subjecting normal RBC in viscous suspending medium (20% dextran) to elliptical deformation induced by application of shear stress under physiologic conditions (290 mOsm/L, 37°C, pH 7.40) in the presence of ouabain and furosemide. Measurement of resulting net passive K efflux ('K leak') demonstrated that shear-induced RBC deformation causes K leak in a dose-dependent fashion at shear stresses far below the hemolytic threshold, an effect shown to be due to deformation per se. To model the specific hypothesis that oxidatively perturbed RBC membranes would be normally susceptible to this potentially adverse effect of deformation, we treated normal RBC with the lipid peroxidant t-butylhydroperoxide. Under conditions inducing only minimal K leak due to either oxidation alone or deformation alone, deformation of peroxidant-pretreated RBC showed a markedly enhanced K leak (P < .001). This highly synergistic oxidation-plus-deformation leak pathway is less active at low pH, is neither chloride-dependent nor calcium-dependent, and allows K efflux to be enhanced by Na influx so there is no change in total monovalent cation content or cell density. Moreover, it is fully reversible since deformation-induced K leak terminates on cessation of shear stress (even for oxidant-treated RBC). Control experiments showed that our results are not explained simply by hemolysis, RBC vesiculation, or development of prelytic pores. We conclude that oxidation and deformation individually promote passive leak of monovalent cation through RBC membranes and that a markedly synergistic effect is exerted when the two stresses are combined. We hypothesize that these findings may help explain the abnormal monovalent cation leak stimulated by deoxygenation of sickle RBC.