Our laboratory has previously studied the cellular and molecular processes involved with bone defect regeneration in a non-geriatric adult rat model. These processes, however, have not been well studied in the aged rat model. This study was designed to evaluate the effects of advanced age on calvarial defect healing in rats. The hypothesis was that bone regeneration in calvarial defects of aged animals would be impaired compared to the 6 month-old adult rat. Bilateral parietal calvarial defects (4-mm; less than critical-sized) were created in adult (6 month-old) and aged (24 month-old) rats. The disk of bone from the left parietal region was placed into the contralateral defect and served as a positive control. The unrepaired left parietal craniotomy site was the study defect. Outcomes from at least n=6/group were measured at 5, 7, 14 and 21 days post-operatively by immunocytochemistry and histomorphometry. These techniques evaluated cellular constituents of the spontaneously healing defects and new bone formation, respectively. Data were subjected to an ANOVA and post-hoc multiple comparison tests. Statistical significance was established at p<0.05. At all time points analyzed, immunolabelling of presumptive osteogenic cells revealed significantly fewer cells in the aged animals compared to the 6 month-old adult rats. By day 14, new bone formation was observed in the calvarial defects of the 6 month-old rats. However, no new bone was detected in the 24 month-old rats at 14 days. Even by day 21, there was no evidence of new bone formation in the 24 month-old rats. Instead, the gap defects were filled with fibrous connective tissue. The data presented provides evidence that spontaneous bone healing is impaired in calvarial defects of aged rats.