The senescence-accelerated mouse (SAM) exhibited a shortened life span (about 18 months) and early manifestation of various signs of senescence, including changes in physical activity, skin, and spinal curvature. The mechanism of senescence acceleration in SAM is thought to be related to free radical damage. Oxidative phosphorylation was estimated in liver mitochondria from SAMPS and the senescence-resistant subtrain, SAMR1. The respiratory control ratio decreased during aging, and the ATP/O, an index of ATP synthesis, was depressed at 18 months of age in SAMPS. DNP-dependent uncoupled respiration in liver mitochondria was markedly decreased, and active uptake of calcium was markedly dysfunctional with aging. These findings suggest that the functional disorders in mitochondria may be closely related to the shorter life span of SAMPS. White-footed (WF) mice can live at least to 5.5 years, when some animals are still capable of reproducing and their external body condition remains healthy. The mitochondrial functions were examined in the same way as in the SAM experiments. However, no particular finding responsible for their longevity was observed in WF mice at 3 and 12 months old. More comprehensive examinations on more aged WF mice are needed for explanation of their greater longevity.