The healing process after implantation of sintered bone in the rat parietal bone was compared with that of synthetic hydroxyapatite using both scanning electron and light microscopy. The results showed that the differences between the sintered natural bone and the synthetic hydroxyapatite implantations were in the states of bone union and the bioresorbability of the implanted materials, even though both materials consist of the same hydroxyapatite. In the sintered bone implantation, the newly formed bone invaded into the material at 1 to 2 weeks after implantation. The sintered bone surface on the dura mater side was completely covered by the new bone at 5 weeks. It is noteworthy that bone resorbing areas characterized by Howship's lacunae were observed on the sintered bone surface at 2 weeks and the material was replaced by new bone. Light microscopy, which revealed the invasion and the development of the new bone into the sintered bone, supported the scanning electron microscopic observations. In the synthetic hydroxyapatite, the new bone adhered closely to the material just like the sintered bone implantation. The new bone did not invade into the synthetic hydroxyapatite. There was no evidence of the resorption of the hydroxyapatite. This shows that the natural and the biological structures of the sintered bone offer an advantageous environment to fluid circulation and ingrowth after implantation.