The rabbit fibula was used as the experimental model in this study of the biological changes in nonvascularized and vascularized bone grafts (N = 72). Radiological study showed that the time required for union was about the same for both types of graft. Histological study with tetracycline as a marker showed that there were cellular changes in these two types of graft: moderate hematopoietic and osteocytic loss associated with an increase in bone porosity in vascularized grafts and virtually total osteocytic renewal and marrow necrosis in nonvascularized bone grafts. Quantitative study of vascularization by flow measurement using labeled microspheres showed a brief initial phase of hypervascularization in vascularized grafts, while revascularization took place early in nonvascularized grafts and was followed by hyperemia from the first to the third month. Calcium uptake curves for the two types of graft were quite closely superimposable upon blood flow rate curves. However, the comparative results presented in this study may be tempered by the fact that all grafts were applied to a recipient bed of excellent quality, consisting of well-vascularized muscles.