We detected nerve growth factor (NGF) by immunohistochemical localization in both fractured and unfractured rat rib. In unfractured bone, periosteal mesenchymal osteoprogenitor cells appeared to be the only skeletal cells which stained for NGF. Adjacent skeletal muscle fibers exhibited NGF staining both in fractured and unfractured bone. Fracture callus periosteal osteoprogenitor cells, marrow stromal cells, osteoblasts, young osteocytes and endothelial cells of new capillaries had moderate to heavy staining for NGF at 1 and 3 weeks after fracture. Deeply positioned osteocytes and osteoclasts showed no NGF staining. Most chondrocytes of fracture calluses stained for NGF, however, some chondrocytes did not stain which may indicate that NGF is produced at particular stages of chondrocytic differentiation. In calluses, periosteal matrix stained heavily for NGF when juxtaposed to cartilage and less obviously when associated with new bone at both 1 and 3 weeks post-fracture. However, other fibrous, cartilaginous and osseous matrices did not stain for NGF at any time. At 6 weeks post-fracture, NGF staining was largely confined to periosteal osteoprogenitor cells. The detection of NGF in periosteal osteoprogenitor cells of unfractured rib points to these cells having a role in nerve maintenance in intact bone. Furthermore, the localization of NGF in osteoprogenitor cells, marrow stromal cells, osteoblasts, certain chondrocytes, endothelial cells, periosteal matrix of the fracture callus and skeletal muscle may mean that these entities participate in fracture innervation. The presence of NGF in the callus may also indicate a direct, as yet undefined action of this neurotrophin on skeletal cell metabolism.