1. The purpose of this study was to determine whether biochemical changes of skeletal muscle that occur as a result of exercise in young rats persist into adulthood. 2. Littermates (10 days old) were assigned to a 3, 6 and 12 week control or training group. In addition, a rest-exercise group (R-E) and exercise-rest (E-R) group were included. 3. The rest-exercise and exercise-rest rats were maintained for the 12 weeks with the first 6 weeks being either rest or exercise and the condition reversed during the last 6 weeks of the experiment. 4. Myofibril ATPase activity of rat plantaris increased from the 10d to 12 week animals (P less than 0.05). As anticipated, training resulted in a lowered activity at 6 and 12 weeks compared to controls. 5. The Ca2+ uptake and Ca2+-ATPase activity of the sarcoplasmic reticulum followed a similar pattern. 6. With regard to the exercise-rest rats, the myofibril and SR ATPase activities at 12 weeks were comparable to the 12 weeks control rats. 7. The rest-exercise group approximated the 12 week training group with regard to myofibril and SR ATPase activities (P less than 0.05). 8. The results suggest that the training adaptations that occur during development of skeletal muscle return to normal, when training ceases in the adult rat. 9. Furthermore, animals that started to train prior to puberty do not have a greater capacity to adapt than animals which initiated training during adulthood.