The aim of this pilot study is to obtain preliminary in vivo data on the use of Calcium Phosphate Cement (CPC) as a root canal filling material. CPC has the potential of being an ideal root canal filler-sealer because it has many advantages including: high biocompatibility, good dimensional stability, good adaptation to the canal walls and good working time. A preliminary in vitro study showed that CPC is injectable into the root canal of an extracted tooth. The anterior, premolar and molar teeth of four cynomolgus monkeys will be utilized. The teeth in the left quadrants will be filled with CPC and the opposite control quadrants filled with gutta-percha using standard endodontic procedures. All teeth will be radiographed before treatment, immediately after filling and before sacrifice. Two animals will be sacrificed at the end of three months post-operatively and the remaining two monkeys sacrificed at the end of six months post-operatively. The harvested teeth embedded in the maxillae and mandibles will be fixed and processed for light and electron microscopic evaluation to determine the histocompatability of the CPC with the surrounding periapical tissues. The properties of CPC to be evaluated in this study will include: (1) sealing ability of the apical foramen and accessory canals, (2) dimensional stability in in vivo conditions, and (3) adaptability to the root canal surface, (4) periapical tissue reactions in case of under-or overfilling of the root canal. Quantitative data will be analyzed for significance using a two tailed "t" test. The data may permit evaluation of CPC as a more ideal root canal filling material. Data from this pilot study will be used to determine the feasibility of evaluating CPC as a root canal filling material in a larger number of monkeys over a longer experimental period. Eventually, it is intended, if data supports the effort, to initiate clinical trials of CPC.