Sequences for 12 protein-coding genes, 2 rRNA genes, and 21 tRNA genes (except for lysine tRNA) were collected from mitochondrial DNAs of various mammals and some other tetrapods, with which pairwise molecular distances were estimated and plotted against probable divergence times. For each gene, pairwise distances corrected for the rate heterogeneity among sites increased in good relation to the divergence times. The rate of sequence changes differed from gene to gene but even the fastest ATPase subunit 8 gene showed no sign of saturation. These results provide the possibility that, although mitochondrial genes accumulate sequence changes rapidly and the rate differs consid-erably between individual genes, one can use mitochondrial genes to correctly estimate molecular distances and thereby divergence times for ancient divergences that occurred more than 100 million years ago.