C57BL/6J mice (6, 19 and 27 months old) were trained in an electrically driven treadmill for a total period of 10 weeks. The training program started with 5 min. run per day and reached a maximum of 30 min. per day. Light and electron microscopy examinations of the gastrocnemius muscle showed that in young animals endurance training did not evoke any appreciable changes. In contrast, both test animals and their controls in the old group revealed clear signs of muscle cell atrophy which for the most part was accompanied by focal loss of myofilaments, mitochondrial changes and an increase in the amount of endomyseal collagen fibrils. In the old-trained group, the most obvious abnormality comprised of multiplication of both the T-tubules and of the tubules of the sarcoplasmic reticulum. Consequently, most of these animals showed the formation of tubular aggregates in their gastrocnemius muscle. Hence, it appears that endurance training in old animals tends to enhance the regressive changes that accompany the aging process of mammalian skeletal muscle.