The rates of restitution of skeletal muscle, heart, and brain creatine phosphate and brain ATP, following experimental depletion, were compared in young (3–5 month), adult (12 month) and old (24 month) rats. In skeletal muscle, restitution of creatine phosphate after 3 min of recovery was greatest in the young rats, minimal in the adult animals, and absent in the old animals. In heart muscle, on the other hand, restitution was rapid in all three age groups, and was essentially complete after 60 sec recovery. In brain, restitution of creatine phosphate was rapid and was complete after 10 min recovery in the young and adult rats; restitution was slower and less complete in the old rats. In the old rats there was no restitution of brain ATP during the first 2 min of recovery, after which the rate of restitution paralleled that of the young and adult rat