Fast-twitch plantaris muscles of female rats were subjected to unilateral compensatory overload, induced by partial excision of synergistic muscles. One group of rats remained sedentary whereas another was subjected to a supplemental program of treadmill exercise consisting of walking 3 m/min, 65% grade, 2 h/day, 5 days/week. Groups of rats were sacrificed after 1, 2, 4, and 8 weeks and their muscles were weighed and analyzed for protein, citrate synthase, phosphofructokinase (PFK) and myofibril ATPase. Absolute and relative (muscle weight/body weight) muscle weights were much greater in both overloaded groups as compared to contralateral controls. However, treadmill exercise also increased the absolute and relative muscle mass of control plantaris muscles in the exercising group as compared tonormal sedentary contralateral controls. Citrate synthase activity was decreased in overloaded, sedentary muscles as compared to contralateral controls, but after 8 weeks of exercise, it returned to normal levels. PFK was decreased in both sedentary and exercised overloaded muscles throughout the 8 week period. Myofibril ATPase showed a tendency to be reduced in sedentary, overloaded muscles, and was significantly reduced in overloaded, exercising muscles. These results collectively suggest that certain fibers of overloaded fast-patterns take on similar in certain aspects to that normally seen in differentiated slow-twitch muscle fibers.