The cellular mechanisms underpinning changes in metabolism during postnatal development in young mammals have not been extensively examined. This study examines changes in sodium pump capacity (Na+, K+-ATPase activity), number and molecular activity, as well as, Na+ flux, cholesterol level and fatty acid composition in a number of major organs during postnatal development in the rat. In liver, Na+ flux was highest (2.6 times) in the youngest rats (3-day old) and decreased with increasing age, whereas Na+, K+-ATPase activity increased with age (up to 9–28 days) in liver, kidney and brain, but not in heart. Increases in Na+, K+-ATPase activity where primarily driven by increases in molecular activity, 4-fold in brain and 7-fold in kidney, rather than by increases in sodium pump number. Membrane polyunsaturation increased in both kidney and brain during development, with kidney becoming increasingly dominated by omega-6 (18:2n-6 and 20:4n-6) and brain by omega-3 (22:6n-3) fatty acids. Membrane reconstitution experiments support the concept that changes in membrane composition might underpin higher sodium molecular activities in the adult. In conclusion, at birth rats possess high Na+ flux but a lower sodium pump capacity that increases with age being driven by increases in molecular activities associate with changes in membrane lipid composition.