Carbohydrate and lipid substrate changes associated with fasting were similar in aged (over 24 months), 1-year-old and young (about 4 months old) rats. In all three age-groups fasting reduced liver and skeletal muscle glycogen, elevated myocardial glycogen and plasm free fatty acid (FFA) levels, but did not significantly affect blood glucose. With refeeding, the myocardium from aged and 1-year-old rats lacked the glycogenesis observed in young rats. Less glycogenesis was also observed in aged soleus muscles during refeeding than in soleus muscles from young or 1-year-old rats. This depressed glycogenesis in the old rats could not be attributed to any change in the tissue triglyceride or plasma FFA response to refeeding, but was accompanied by a slightly greater elevation of glucagon in the aged rats. Though its etiology is unclear, the depressed glycogenesis indicates that aging affects aspects of carbohydrate metabolism in addition to the known decrease in glucose tolerance.