1. An increase in the intracellular concentration of inorganic phosphate (Pi) reduces the rate of the Na:K exchange catalysed by the Na pump in red cells. The inhibitory effect of Pi is exerted on the maximum rate of flux, Pi having no appreciable effect on the apparent affinity of the Na pump for either internal Na or external K. The effect of Pi is exerted along a rectangular hyperbola which tends to zero as Pi tends to infinity and is half-maximal at about 17 mM internal Pi. 2. Pi does not modify the rate of Na:Na exchange catalysed by the Na pump. 3. A reduction in the intracellular concentration of ATP reduces the maximum rate of Na:K exchange having no effect on the apparent affinity for either internal Na or external K. 4. The effects of ATP and Pi are mutually independent. 5 The lack of effect of ATP and Pi on the apparent affinity for internal Na is compatible with the idea that the affinity of the inner sites of the Na pump remains constant during a pump cycle. 6. The lack of effect of ATP on the apparent affinity for external K and the independence between the effects of ATP and Pi are difficult to explain if the only effect of ATP were its combination at a phosphorylating site. 7. The apparent affinities for K and phosphate become independent of the concentration of ATP, if it is assumed that in our experimental range the phosphorylating site is fully saturated with ATP, the rate of pumping being controlled by the state of occupation of a second non-phosphorylating site whose affinity for ATP is much lower. 8. The lack of effect of Pi on the apparent affinity for external K seems to indicate that during Na:K exchange the conformations of the pump that predominate are endowed with a reactivity towards inorganic phosphate and have the same high affinity for K in both their phospho and their dephospho states. 9. The kinetic behaviour of the Na pump in regard to its interactions with inner and outer cations, ATP and Pi seems to indicate that, in contrast with what happens with soluble allosteric proteins, in the active transport system ligand-induced changes in the reactivity are more important than ligand-induced changes in affinity. In this respect therefore the Na pump behaves as an allosteric 'V system'.