Recently, the mechanism of action of prostaglandins in promoting bone resorption has received wide attention. Prostaglandins are used in orthodontics to shorten the period of tooth movement. PGE1 was administered locally (5 micrograms/kg every 12 hours) and systemically (7.5 ng/kg/min) to rats to study the difference in the efficacy of the two methods of administration in accelerating bone resorption. The Waldo method was used on 72 Wistar rats (8 weeks old) for 5 days to induce mesial movement of the first molar. The body weight of the rats was measured every 12 hours from the start of the experiment. Tissues were examined in the mesial surface of the mesial root and in the interradicular septum between the linguomesial root and the linguodistal root of the upper first molar. The pressure side of the mesial surface of the mesial root was studied, as well as the pressure side of the interradicular septum. The results showed that decrease in body weight during the experiment was more prominent in the local administration group than in the systemic administration group. In both the group receiving PGE1 through local administration and the group receiving PGE1 through systemic administration, the numbers of osteoclasts and Howship's lacunae were markedly increased over those in the control groups. Moreover, compared with local administration, systemic administration of PGE1 had a more marked effect on bone resorption.