Senescence involves a decline in physiological performance with age paralleled by increased mortality and decreased reproductive rate. According to the "free radical theory of living" the main cause of senescence, and eventually death, is the accumulated damage caused by reactive oxygen species (ROS) and other free radicals on cell structures. The cellular production of ROS occurs in itself in direct proportion to the aerobic processes, creating a possible mechanistic link between aging and oxygen consumption. Studies on the relationship between age and metabolic rate have, however, given conflicting results. The basal metabolic rate (BMR) is the lowest sustainable rate of oxidative processes of a resting endothermic organism, which do not use energy on thermoregulation and digestive processes. If a continuous effect of ROS is present throughout life, a steady decrease in BMR should be observed. This is, however, not always observed in endotherms, suggesting the mobilization of an efficient antioxidant enzymatic system in some species. In a longitudinal study we have investigated the relationship between BMR and age in captive zebra finches (Taeniopygia guttata) up to seven years of age. This exceeds the normal life span of wild-living individuals. We found a significant decline in BMR with age in both female and male finches. In addition, we found that individuals with the highest BMR, also showed the largest relative decline in BMR. These results support the free radical theory of living, as the individuals with the highest BMR would be expected to produce more ROS, which in turn would result in more cellular damage. Our results also indicate that these small passerine birds may have traded defense against ROS with energy use for other physiological functions.