This experiment was performed to explore the effect of electric currents and orthodontic forces on bone PGE₂ content and orthodontic tooth movement in cats. Stainless steel electrodes were connected a power pack consisting of five miniature batteries, a transistor, and a resistor. The current (10±2μA) was provided by a constant source encased in a palatal acrylic plate. In first experiment, the cathode was placed mesial to the right maxillary canine tooth and the anode was positioned distal to the tooth, Sham slsctrodes were placed near the left cuspid, to serve as control. Nine cats were divided into three groups evenly. Groups of three animals were treated with electric currents only-for 1,3 and 7 days, respectively. In second experiment, electric currents and the orthodontic forces of about 80 gm were applied to the right maxillary canine, and the orthodontic forces only were apploed to the left maxillary canine. 3 groups of three cats each were treated in this experiment-for 1,3 and 7 days, respectively. Alveolar bone samples were obtained from sites of tension and compression as well as from contralateral sites. Bone samples were extracted by homogenization in 40% ethanal. The supernatant partitioned twice with 2 volumes of petroleum ether to remove neutral lipids and the aqueous supernatant partitioned in ethyl acetate. After drying the solvent, PGE₂ was ,easured by radioimmunoassay techniqye. The obtained results were as follows. 1. Teeth treated with combined force and electricity moved faster than those treated with force alone. 2. Alveolar bon PGE₂ content of electric stimulation was increased at both electrodes. 3. Alveolar bone PGE₂ content of mechanical stimulation at compression sites was gradually increased at all time period. At tension site, PGE₂ content increased after 1 day of mechanical stimulation remained elevated at all time period. 4. Alveolar bone PGE₂ content of compression sites was increased more than that of tension sites from mechanical stimulation as wess as electrical stimulation.