The mRNA level of uncoupling protein (UCP) specific for brown adipose tissue (BAT) in Daurian ground squirrel, was detected by using a [32P]-labeled oligonucleotide probe. The UCP concentration in mitochondria was indirectly determined by titration with its specific ligand [3H]-labeled GTP. Type II T4 5′-deiodinase of BAT was assayed concomitantly. We found two species of mRNA for UCP with lengths of about 1.9 and 1.5 kb, respectively, both occurring in almost the same concentration. UCP mRNA content was elevated significantly during hibernation, but the UCP concentration did not change compared with that of nonhibernating controls kept at room temperature. When hibernating squirrels were aroused, the UCP mRNA remained at the elevated level as during hibernation, but the UCP concentration increased in comparison with that of nonhibernating controls or during hibernating. Changes in T4 5′-deiodinase activity in BAT were similar to the variations of the UCP mRNA level. These results suggest that the activation of T4 5′-deiodinase in BAT may be an important factor for the up-regulation and maintenance of UCP mRNA content needed for the synthesis of sufficient UCP to acquire the thermogenic capacity for arousal from hibernation. © 1998 Elsevier Science Inc. All rights reserved.