Male rats were either sham-operated (N) or castrated (C) at 65 days of age. They were further subdivided into sedentary or exercise groups that were trained by treadmill running 5 days/wk for 12 wk. During the last 10 days of training, the animals received daily subcutaneous injections of cortisone acetate (CA) (100 mg/kg) or 1% carboxymethylcellulose. Body weight decreased approximately 25% in all groups that received CA. The fast-twitch plantaris and gastrocnemius muscle weights were approximately 35% lower in CA-treated versus cortisone-free N and C sedentary animals. Exercise prevented from one-fourth to one-half of the muscle weight loss in N and C runners when compared to their respective pair weight controls. Muscle weights of the CA-treated freely eating N controls were significantly less than that of N runners that received CA. In plantaris muscles of both N and C animals that received CA, total protein concentration and citrate synthase activity, a mitochondrial marker, remained constant, but their amounts per muscle decreased in proportion to the atrophy. However, myoglobin concentration increased in plantaris muscles of CA-treated animals, although total myoglobin per muscle was reduced slightly. Myoglobin levels were increased in plantaris muscles both as a result of training and CA, but citrate synthase activity was increased only as a result of the exercise. These results show that exercise can retard the glucocorticoid-induced muscle atrophy.