Twenty Sprague-Dawley rats weighing 280–300 g were divided into two groups often animals each.They were treated by daily submucosal injections of 50 µg prostaglandin E2 (PGE2) per kilogram body weight into the region below the apex of the left first maxillary molar (experimental) or vehicle into the region below the apex of the right first molar (control), for a period of 5 days.The animals of the first group were sacrificed immediately following the treatment period, while those of the second group were sacrificed 5 days after the treatment period.Twenty-two hours prior to sacrifice, a piece of latex orthodontic elastic was secured to the adjacent area between the first and second maxillary molars of both sides of each rat by using two mosquito hemostats.The periodontál ligament (PDL) mesial to the mesiobuccal root of the first maxillary molar was assayed for changes in PDL cell factors. The results showed that immediately following the 5-day treatment period the left PDL had a significant decrease in the total number of fibroblasts and a significant increase in the total number of both osteoclasts and nuclei per osteoclast, while no significant changes in the osteoblasts when compared with those of the right control PDL. The left PDL of animals which were sacrificed 5 days after the treatment period revealed a significant decrease in the number of total fibroblasts and only a slight decrease in both numbers of total osteoclasts and total nuclei per osteoclast, but again no significant changes in osteoblasts when compared with those of the right control PDL. The reduction in the number of total fibroblasts in the left PDL was attributable to the submucosal injection of PGE2 which probably inhibited mitoses of fibroblasts. In addition, submucosal injection of PGE2 might also have induced transformation of monocytes into osteoclasts and an increase in the osteoclast-resorptive activity, although its effect is short-acting. All the above findings demonstrated that treatment with 50 µg PGE2/kg/day for 5 consecutive days inhibited the proliferation of PDL fibroblasts and activated alveolar bone resorption during orthodontic tooth move