The abundance of the mitochondrially encoded mRNA for subunit II of cytochrome c oxidase (COII mRNA) increases in the liver of rats exposed to environmental cold stress (4 degrees C ambient temperature). Only transient increases or no changes in COII mRNA levels were observed in brown fat and soleus muscle respectively. The time course of the liver COII mRNA increase was compared with the effects of cold stress on mitochondrial 16S ribosomal RNA expression and indicated that cold induces a rapid (few hours) increase in the liver mitochondrial mRNA levels and high levels of both messenger and ribosomal RNA mitochondrial transcripts are present after a few days of cold exposure. No changes in mitochondrial DNA abundance relative to total cellular DNA were observed in the liver of rats at any time during cold stress. It is concluded that mitochondrial genome expression is specifically increased in the liver of cold-exposed rats through different mechanisms, independent of changes in mitochondrial genome abundance.