Responses of aging brain to physical training was evaluated by quantifying the substrates, glucose, lactic acid, and nucleic acids in cerebral cortex (CC) and medulla oblongata (MO) of the brain in rats. Rats of 1 month (young), 6 months (adult), 12 months (middle-aged) and 18 months (old) of age were swim-trained for 30 days. Glucose content of CC and MO increased with training whereas blood glucose decreased in trained young and adult animals with middle-aged and old animals maintaining constant blood glucose. Brain lactate in these two regions decreased with training in all age groups. However, the old animals showed an elevation in blood lactic acid in trained state, while the other age groups showed a decrease. Nucleic acid content, decreased with age, especially the RNA content in MO showing a larger depletion. However, there was no discernible influence of physical exercise on these parameters. Physical training has influenced the aging brain's adaptability, as seen by increase in its glucose content in young animals and also possible utilization of lactate as an additional substrate in old animals as evidenced by an increase in blood lactic acid.