Red cell Na+ and K+ content and transport were studied in Sprague-Dawley rats in the course of a dietary K+ depletion ranging 1-6 wk. Plasma K+ fell to below 2 mM, and red cell K+ decreased. Cellular Na+ rose due to an increase of the Na+ leak. Inward Rb+ and outward Na+ transport by the Na+-K+ pump (determined at 2 mM external Rb+) were accelerated by the rise in cell Na+ concentration. K+ depletion caused a cation deficit of up to 30% of total red cell Na+ plus K+ and a consecutive cell shrinkage with an increase in mean cellular hemoglobin content (MCHC). The cell shrinkage, in turn, was paralleled by up to a 10-fold increase in the maximum capacity of the furosemide-sensitive, chloride-dependent Na+-K+ cotransport system. This system participated with up to 50% of the total K+ movements across the red cell membrane in severe K+ deficiency. In normal cells shrunken by osmotic means, Na+-K+ cotransport was similarly accelerated severalfold, indicating that the cell shrinkage occurring during K+ depletion is a major factor inducing the changes in Na+-K+ cotransport. However, a second unknown factor is also involved. It is concluded that in the rat, not only genetic but also environmental parameters contribute in determining the actual activity of the red cell Na+-K+ cotransport system. The cell volume and MCHC must be considered when judging Na+ and K+ transport changes observed in rat erythrocytes under various pathophysiological conditions.