Cellular sodium transport via the Na+, K+-ATPase contributes significantly to daily energy expenditure. The effect of a meal on leucocyte Na+ transport and intracellular electrolytes was therefore investigated in lean normal subjects at room temperatures of 23" and 33", to determine if the Na pump responds to the need for thermogenesis. In the fasting state, the ouabain-sensitive efflux rate which reflects active Na' transport, and the intracellular electrolytes were similar. At 2 h after eating a 4.2 MJ (1000 kcal) meal, the ouabain-sensitive efflux rate constant rose when the room temperature was 23" but not at 33". The ouabain-sensitive Na+ efflux rate, an index of active Na+ transport, rose post-prandially at 23" only. The post-prandial activation of leucocyte active Na efflux in normal subjects was blunted at higher environmental temperatures, when the need for thermogenesis was reduced.