There is a need for new therapeutic agents designed to prevent or restore skeletal muscle loss in frail, elderly subjects resulting from injury or disease and associated catabolic stresses such as malnutrition. Since the beta 2-adrenergic agonist clenbuterol increases skeletal muscle mass in rats, the effect of this agent on recovery of muscle mass and carcass protein content following protein malnutrition was studied in young and old rats. The 3-week period of severe dietary protein restriction reduced body weight 21%, hind-limb muscle weight 24 and 15%, and carcass protein content 31 and 19%, respectively, in 3- and 24-month-old rats. During the 3-week recovery period induced by feeding a complete diet, 10 mg clenbuterol per kg diet increased hind-limb muscle weight 34 and 30% and carcass protein content 27 and 25%, respectively, in 3- and 24-month-old rats. Restoration was complete in animals of both ages fed clenbuterol and incomplete in animals fed the control diet. These observations suggest that clenbuterol or similar beta 2-adrenergic agonists may be useful in hastening the recovery of muscle mass and body protein stores lost because of malnutrition in frail, elderly humans.