Abstract There is currently no satisfactory explanation for caloric restriction's (CR's) well-established ability to reduce the rate of aging. Pearl's rate of living theory postulates a direct relationship between metabolic rates and the rate of aging, but CR, exercise and cold-exposure studies have shown that this theory is not valid in rodents with respect to whole-body energy metabolism. The present paper describes a crude analysis of previously published data from rat CR, wheel running and cold exposure studies, which reached two main conclusions. The first is that there appears to be a direct relationship between organ metabolic rates and the rate of aging. The second is that organ basal metabolic rates (BMRs) decrease by about one-quarter during adulthood in the rat. On the basis of these findings, the following two hypotheses are proposed: (1) the rate of living theory is valid in mammalian organ; (2) organ BMRs decrease by about one-quarter in adulthood in mammals. This hypothesized decrease, if confirmed in humans, would be a major component of the well-established age-related decrease in whole-body resting metabolic rate in humans.