BACKGROUND: Studies show that proper exercise has the effect of anti-aging, increasing bone mass accumulation and reducing bone mass loss. OBJECTIVE: To investigate the effect of aerobic exercises on osseous composition In aging rats. DESIGN, TIME AND SETTING: A randomized control animal experiment was completed in the Animal Experiment Laboratory of Xi'an Sports Institute from April to June in 2007. MATERIALS: A total of 64 clean male SD rats weaned for 2 months were randomly divided into a normal control group, a non-modeling exercise group, an aging model group and a modeling exercise group, with 16 ones in each group. Every 8 rats of each group were sacrificed at the 6th and 9th week respectively.exercise for 6 and 9 weeks. MAIN OUTCOME MEASURES: At the end of week 6 and 9, rats in each group were detected regarding to the following measures: serum alkaline phosphatase, bone gla protein, serum propeptide of type] procollagen, serum total calcium, serum inorganic phosphorus, serum magnesium, urinary calcium, urinary inorganic phosphorus, urinary magnesium, urinary hydroxylysine glycoside. Bone trabecula changes in femoral bone tissue were observed with light microscope. RESULTS: Compared with the normal control group, the aging model group got a decrease at the following indexes at both week 6 and week 9, namely, serum alkaline phosphatase, serum propeptide of type Ⅰ procellagen, serum total calcium, serum magnesium, urinary calcium, urinary inorganic phosphorus, urinary hydroxyproline. While in the non-modeling exercise group and the modeling exercise group, these above-mentioned indexes increased. Compared with the normal control group, the aging model group got an increase at the following indexes at both week 6 and week 9, namely, serum osteocalcin, inorganic phosphorus, adnary magnesium, urinary hydroxylysine glycoside. While in the non-modeling exercise group and the modeling exercise group, these above-mentioned indexes decreased. In Addition, exercise of 9 weeks led to a better recovery result than that of 6 weeks. The detection under light microscope showed an obvious few bone trabeculas in the aging model group and an increase of bone trabeculas per unit volume in the non-modeling exercise group and the modeling exercise group. CONCLUSION: Aerobic exercise has a great effect on the morphosis of femoral bone osseous composition in aging rats, and it can delay osteoporosis.