A number of observations have been made to examine the role that mitochrondrial energetics and superoxide anion production play in the aging of wild-type Caenorhabditis elegans. Ultrastructural analyses reveal the presence of swollen mitochondria, presumably produced by fusion events. Two key mitochondrial functions - the activity of two electron transport chain complexes and oxygen consumption - decreased as animals aged. Carbonylated proteins, one byproduct of oxidative stress, accumulated in mitochondria much more than in the cytoplasm. This is consistent with the notion that mitochondria are the primary source of endogenous reactive oxygen species. However, the level of mitochondrially generated superoxide anion did not change significantly during aging, suggesting that the accumulation of oxidative damage is not due to excessive production of superoxide anion in geriatric animals. In concert, these data support the notion that the mitochondrial function is an important aging determinant in wild-type C. elegans.