Purpose: Although the recent success rates of dental implants are quite high, it is still true that loading during the bone healing period may be more likely to discourage osseointegration and that occlusal overloading might result in implant failure, even after osseointegration is established. The purposes of this study were to establish an appropriate experimental animal model and to histologically assess degenerative changes in established osseointegration under early and excessive occlusal loading. Materials and Methods: Forty rats were divided into control and experimental groups. The maxillary first and second molars on both sides were extracted, and machined-surface titanium implants were placed. In the experimental group, 2 or 4 weeks after implant placement, abutments that were designed to overload the implants were attached. Control group implants did not receive abutments and remained in situ 2 or 4 weeks. Sections were prepared and observed histologically. Results: Attrition of occluding opposite teeth and shiny spots on the abutments indicated that this model was useful for histologic investigation of the remodeling and bone changes around implants. Specimens showed remarkable bone loss and deterioration of osseointegration when overloading began at 2 weeks. Overloading applied after 4 weeks of healing induced active bone resorption in remote areas of the implants after 15 days of occlusion, while bone resorption at the interface was limited. Conclusion: The authors successfully established an implant occlusion model using rats. This model revealed degenerative changes in osseointegration and/or in the bone around implants upon excessive occlusal loading. These results emphasize the risks associated with immediate loading and overloading. This is the first study to reveal the possibility of bone loss around overloaded implants in the absence of infection using a small animal model. Keywords: animal experiments, bone remodeling, dental implants, histology, occlusal overload, osseointegration