AbstractOur previous studies have suggested that synthetic octacalcium phosphate (OCP) could be resorbed and replaced by newly formed bone if implanted in rat skull defects. We hypothesized that the implanted OCP is more resorbable than other commonly used bone graft substitutes of calcium phosphate compounds, such as hydroxyapatite (HA) and β‐tricalcium phosphate (β‐TCP). To test the hypothesis, the present study was designed to compare histomorphometrically resorption of the implanted OCP, HA, and β‐TCP, which were kept in the experimental cranial defect of rats for a long term. A full thickness of standardized trephine defect was made in the rat parietal bone, and the same volume of granules of OCP, HA, and β‐TCP were implanted into the defect. Five specimens of each group were fixed 6 months after implantation. The percentage of remaining implants (r‐Imp%) and newly formed bone (n‐Bone%) in the defect was analyzed histomorphometrically. The statistical analysis showed that the r‐Imp% of OCP was significantly lower than that of HA and β‐TCP. In contrast, the n‐Bone% of OCP was significantly higher than that of HA and β‐TCP. The present study has shown that the implanted OCP in the rat cranial defect is more resorbable than the implanted β‐TCP and HA, whereas the implanted OCP enhances bone formation more than the implanted β‐TCP and HA. © 2001 Wiley Periodicals, Inc. J Biomed Mater Res 59: 29–34, 2002