The evolutionary rate of mitochondrial DNA (mtDNA) is highly variable across lineages in animals, and particularly in mammals. This variation has been interpreted as reflecting variations in metabolic rate: mitochondrial respiratory activity would tend to generate mutagenic agents, thus increasing the mutation rate. Here we review recent evidence suggesting that a direct, mechanical effect of species metabolic rate on mtDNA evolutionary rate is unlikely. We suggest that natural selection could act to reduce the (somatic) mtDNA mutation rate in long-lived species, in agreement with the mitochondrial theory of ageing.