Na+-ATPase of high-K+ and low-K" sheep red cells was examined with respect to the sidedness of Na+ and K+ effects, using inside-out membrane vesicles and very low ATP concentrations (less than or equal to 2 muM). With varying amounts of Na+ in the medium, i.e., at the cytoplasmic surface, Na+cyt, the activation curves show that high-K+ Na+-ATPase has a higher affinity for Na+cyt compared to low-K+. The apparent affinity for Na+cyt is also increased by increasing the ATP concentrations in high-K+ but now low-K+. With Na+cyt present, Na+-ATPase is stimulated by intravesicular Na+, i.e., Na+ at the originally external surface, Na+cyt, to a greater extent in low-K+ than high-K+. Intravesicular K+ (K+ext) activates Na+-ATPase in high-K+ but not in low-K+ vesicles and extravesicular K+ (K+cyt) inhibits low-K+ but not high-K+ Na+-ATPase. Thus, the genetic difference between high-K+ and low-K+ is expressed as differences in apparent affinities for both Na+ and K+ and these differences are evident at both cytoplasmic and external membrane surfaces.