We found opposite regulation of uncoupling protein 3 (UCP3) in slow-twitch soleus and fast-twitch gastrocnemius Muscles Of rats during cold exposure. Namely, the UCP3 mRNA level was downregulated in the soleus muscles, but upregulated in the gastrocnemius Muscles after a 24-h cold exposure. In the analysis of UCP3 protein, we first succeeded in detecting UCP3 short-form as well as the long-form in vivo, which levels were decreased markedly in the cold-exposed soleus Muscles. However, the levels of UCP3 and cytochrome oxidase Subunit IV were well maintained in the cold-exposed gastrocnemius muscles with a rise in the total mitochondrial protein level, suggesting an increase of total oxidative ability. The fast-twitch muscle rather than the slow-twitch one may play an important role in adaptive responses, including thermogenesis under acute cold exposure. (C) 2003 Elsevier Science (USA). All rights reserved.