Bone defects that cannot be healed completely are termed critical-sized defects and can be used to test bone grafts for medicine, dentistry, and periodontology. The aim of the present study was to detect the effects of a xenograft (Unilab Surgibone) on bone building in experimentally created parietal bone defects in rats. Standardized parietal bone defects were created in 16 rats, and each defect had a circular morphology 6 mm in diameter. The right defect sites were filled with porous particle material, and the left site was used as control. After the 3rd, 6th, and 12th months, rats were killed and tissue samples obtained from the related site of the cranium. Subsequently, histological sections were taken and stained with different stains for evaluation under light microscope. The rate of bone formation was assessed using a senniquantitative method. These results showed that dense collagenous tissue was observed in the control area during the third month, whereas xenograft particles were surrounded by a fibrous tissue layer at the implantation site. Osteoclast-like cells were also observed. There was also no significant bone repair at other observation periods. It can be concluded that the material used had no evidence of resorption and does not enhance bone formation. However, it seems biocompatible, osteoconductive, and could be used in a limited manner as a material for filling osseous defects in clinical practice.