Summary This study aimed to investigate effects of exercise and immobilization just before the exercise on bone remodeling processes. In experiment 1, rats were used as materials, and were divided into three groups: an exercise after immobilization group that performed 4-week-exercise after 4-day-immobilization （IM-EX）， an exercise group that accomplished 4-week-exercise （EX）， and a control that was fed normally for four weeks （CO）. In experiment 2, rats were also divided into three groups: an exercise after immobilization group that performed 1-week-exercise after 2-day-immobilization （IM-EX）， an exercise group that accomplished 1-week-exercise （EX）， and a control that was fed normally for one week（ CO）. Rats in each group were euthanized at the end of experimental period, tibiae were excised from them and those specimens were observed histologically and morphometrically. Density and thickness of the bone trabeculae increased at primary cancellous bone from immediately after starting of experiments in IM-EX of the experiment 1 and 2. As for secondary cancellous bone, IM-EX of experiment 1 showed remarkable decrease of bone mass once after four days immobilization, and it was necessary for four weeks to reached to the same level as EX by exercise. On the other hand, in IM-EX of experiment 2, the bone mass hardly changed by two-day-immobilization but decreased temporarily after 2 and 4 days of exercise period, and then, it already recovered up to level than EX after 7 days. Many osteoblasts showed positive reaction to immunostaining of RANKL, and many TRAP positive cells were also recognized, in IM-EX of experiment 2. And, bone matrixes indicated positive reactions to immunostaining of TGF-β， in each groups. Through the results mentioned above, it was suggested that active bone formation could be brought by the exercise following the short-term immobilization, because factors like TGF-β that promoted differentiation and activation of the osteoblasts were released from the bone matrix by acute bone resorption.Abstract This study aimed to investigate effects of exercise and immobilization just before exercise on bone remodeling processes. Rats were used as materials, structures of their tibiae were observed through experiment 1 and 2, and following data were obtained. It was nessesory for four weeks that bone mass of the 4-week-exercise after 4-day-immobillization group recovered to the level that of the 4-week-exercise group. The bone mass of the 1-week-exercise after 2- dayimmobilization group decreased at once, but reached higher level than 1-week-exercise group after one week of the experiment. Many RANKL and TRAP positive cells were observed after 2 days of the experiment in the1-week-exercise after 2-day-immobillization group, compared to the control, and the bone matrixes indicated positive reactions to TGF-βimmunostaining. Then, it was suggested that the bone resorption activated, differentiation and activation factors like TGF-β were released from the bone matrixes into bone marrow, and then, this could brought active bone formation, by the exercise following the immobilization
