We used UPLC-Q-TOF MS to analyze hepatic metabolites of rats aged 6, 12, 18, and 24 months; the MS data were processed by partial least-squares discriminant analysis (PLS-DA) to investigate the discrimination among sample groups. Rats were significantly separated with increasing age, except those aged between 6 and 12 months. We identified only 25 of 120 metabolites contributing to the separation: lipid metabolites (glycerol-3-phosphate, linolenic acid, lysophosphatidylcholines [lysoPCs]), energy metabolism intermediates (betaine, carnitine, acylcarnitines, creatine, pantothenic acid), nucleic acid metabolites (inosine, xanthosine, uracil, hypoxanthine, xanthine), and tyrosine. Aging accumulated energy metabolism intermediates, hypoxanthine, xanthine, and 2 major lysoPCs (C18:0 and C22:6). The NAD level and NAD/NADH ratio decreased with age. It was indicated that aging might decrease energy production through β-oxidation because of a decrease in NAD despite the accumulation of lipid energy metabolism intermediates. In addition to energy dysregulation, hypoxanthine and xanthine, which are elevated with age, might accumulate reactive oxygen species in the liver. These results strongly support two aging theories: those of energy dysregulation and free radicals. Additionally, we propose a metabolic pathway related to aging based on these hepatic metabolites. These metabolites and the proposed aging pathway could be used to understand aging and related diseases better, and increase the predictability of aging risk.