No correlation between Na, K-ATPase activity increase in rat kidney and the level of T3 in blood plasma was revealed in course of the rats adaptation to cold. Spironolactone prevented the enzyme activation caused by cold. Actinomycin D, administered to the acclimated rats two days before the experiment, decreased the enzyme activity. Addition of actinomycin D to membrane preparation did not change the enzyme activity. The increase in Na, K-ATPase activity in kidney during adaptation of rats to cold appears to be related to DNA-dependent synthesis of RNA. The enzyme activation was apparently regulated by aldosterone but it was not the T3-dependent process.