The extended longevity of bats, despite their high metabolic rate, may provide insight to patterns and mechanisms of aging. Here I test predictions of the free radical or oxidative stress theory of aging as an explanation for differences in lifespan between the little brown bat, Myotis lucifugus (maximum lifespan potential MLSP=34 years), the short-tailed shrew, Blarina brevicauda (MLSP=2 years), and the white-footed mouse, Peromyscus leucopus (MLSP=8 years) by comparing whole-organism oxygen consumption, hydrogen peroxide production, and superoxide dismutase activity in heart, kidney, and brain tissue. Mitochondria from M. lucifugus produced half to one-third the amount of hydrogen peroxide per unit of oxygen consumed compared to mitochondria from B. brevicauda and P. leucopus, respectively. Superoxide dismutase (SOD) activity did not differ among the three species. These results are similar to those found for birds, which like bats have high metabolic rates and extended longevities, and provide support for the free radical theory of aging as an at least partial explanation for the extreme longevity of bats.