The coenzyme Q (ubiquinone) concentrations of a number of tissues have been determined over the life span of the male laboratory rat. Coenzyme Q increased between 2 and 18 months and decreased significantly at 25 months in the heart and kidney, and the gastrocnemius, oblique and deep aspect (red) vastus lateralis muscles. The coenzyme Q concentration of liver increased over the life span, while it remained relatively constant in brain, lung, and the superficial aspect (white) of the vastus lateralis muscle. Data are also included for organ weights and protein contents of tissues over the life span. The various roles of coenzyme Q in cellular electron transfer and its regulation, energy conservation in oxidative phosphorylation, and its clinical efficacy in diseases of energy metabolism are discussed. It is hypothesized that coenzyme Q serves as a free radical quencher in the mitochondrion, a major site of free radical formation, in addition to its other roles in cellular energy metabolism, and that its cellular diminution may contribute to the loss of cellular function accompanying ageing.