In vivo microcomputed tomography (μCT) enables real-time assessment of bone regeneration. The aim of this μCT and histologic experiment was to assess guided bone regeneration (GBR) around standardized calvarial defects in rats using particulate graft material (Bio-Oss) with and without collagen membranes (CMs). Eighteen female Sprague- Dawley rats aged 6 weeks and weighing 300 g were used. With the rats under general anesthesia, calvaria were exposed and a full-thickness standardized defect was created on the parietal bone. For treatment, rats were randomly assigned to the following three groups: (1) CM group; (2) Bio-Oss group; and (3) Bio-Oss + CM group. Bone volume and bone mineral density (BMD) of newly formed bone (NFB) and remnant bone particles were measured at baseline and 2, 4, 6, and 10 weeks after the operations using real-time in vivo μCT. At 10 weeks, all animals were sacrificed and calvarial tissues were assessed histologically. In the CM group, a significant increase in mean ± standard deviation (SD) BMD of NFB was observed at 6 weeks (0.32 ± 0.02 g/mm3) (P .01) compared with baseline. In the Bio-Oss group, mean ± SD volume (3.03 ± 0.14 mm3) (P .05) and BMD (0.14 ± 0.01 g/mm3) of NFB significantly increased at 6 weeks compared with baseline (P .01). In the Bio-Oss + CM group, mean ± SD volume (0.98 ± 0.19 mm3) and BMD (0.13 ± 0.01 g/mm3) of NFB significantly increased at 4 weeks compared with baseline (P .01). In th Bio-Oss + CM group, mean ± SD volume (3.5 ± 0.7 mm3) and BMD (0.44 ± 0.03 g/mm3) of remnant bone particles were significantly reduced at 10 weeks compared with baseline values (5.8 ± 0.96 mm3 and 1.3 ± 0.02 g/mm3) (P .05). Although histologic analysis revealed NFB in all the study groups, the Bio-Oss + CM group exhibited the most. The results of this study revealed that, in real time, new bone formation starts as early as 4 weeks in standardized calvarial defects undergoing GBR with Bio-Oss + CM, compared with new bone formation at 6 weeks in defects undergoing GBR with Bio-Oss alone.