The effects of exercise training and detraining on bone mass were assessed in young male Wistar rats. The rats were divided randomly into sedentary control (C) and exercise training (T) groups. The T rats were trained for 10 weeks followed by a 10-week detraining period. Training consisted of running exercise on a rodent treadmill at 35 m/min, + 5-degree inclination, 60 min/day, 5 days/week. Training induced significant gain in fat-free dry weight and length of bones (femur, tibia, humerus and coxa) and bone mineral content (femur, tibia and humerus). Histological analysis at the tibial mid-shaft revealed a significant increase in total and cortical areas without a significant change in marrow area in the T group. Bone mass acquired through running exercise was retained for 10 weeks after cessation of training. These results indicate that running exercise leads to increased cortical bone associated with enhanced periosteal bone formation which is also maintained even after stopping exercise training, and suggest that training effects on the skeleton in bone mass level do not diminish immediately after cessation of training.