In this paper, the author observed that in different time the changes of the amount of endogenous prostaglandins in rat alveolar bone under continuous orthodontic force, and confirmed that there are certain amount of PGE1. PGE2 in the rat alveolar bone even without orthodontic force, and the amount is no difference between right and left side. After several hours when the orthodontic force was applied the amount of PGE1 and PGE2 in rat alveolar bone rose significantly. But it is transient and does not last 24 hours.