A prospective study using 46 young adult New Zealand rabbits was designed to evaluate onlay bone grafts to the craniofacial skeleton with respect to embryonic origin (membranous or endochondral), gross morphology (unicortical or bicortical), and orientation (cortex-to-bed relationship). Quantitative and qualitative data were analyzed and contrasted at both periods of evaluation (1.5 and 3.0 months). The embryonic origin of onlay bone grafts to the rabbit snout is significantly correlated with graft surface area, volume, weight, and recipient bed union for up to 3 months postoperatively. Over this interval, membranous bone (calvaria) grafts either persist in their entirety or increase, whereas endochondral bone (iliac) grafts resorb. Neither the number of cortices (unicortical or bicortical) nor the orientation of unicortical grafts (cortex-to-bed relationship) affected graft fate regardless of embryonic origin. Bone density remained unaltered during both resorption and deposition. Oste-ogenesis, demonstrated by serial fluorochrome markers, occurs in both membranous and endochondral bone grafts. Histologically, bone grafts of membranous and endochondral origin differ greatly in their cortical to cancellous diploe ratios and architectural configuration. We hypothesize that the differences found are related to the three-dimensional osseous architecture rather than to the embryonic origin of bone per se.