The present protocol is designed to study the initial cell kinetics and cell population dynamics of orthodontically induced periodontal ligament (PDL) osteogenesis. Bilateral interproximal latex elastics will be placed between the maxillary first and second molars of 6 - 8 week old young adult male rats. This procedure results in a widening of the PDL ("area of tension") and new bone formation within 72 hours on the mesial aspect of the mesial root of the first molar (Roberts and Jee, 1974). At each hourly interval from 0 - 72 hours after activation, two animals will be injected with tritiated thymidine and sacrificed one hour later. Autoradiographs will be prepared from 3 micron serial sections oriented in the saggital plane for maxillary left first molars and in the transverse plane for contralateral specimens. In a standardized sample of midroot PDL, 225 micron in length and width, the following parameters will be quantitated in absolute terms as a function of time following orthodontic activation: 1) labeled and unlabeled PDL cells, 2) mitotic figures, 3) necrotic cells, 4) osteoblasts, 5) new osteocytes, and 6) new bone formation. These methods are expected to yield data of sufficient quality to allow derivation of a meaningful multicompartment kinetic model to help explain the mechanism of stimulated osteogenesis.