After injection of ferritin into the tibial nutrient artery of adult dogs, ferritin was seen in the interstitial fluid compartment of the tibial cortex immediately and up to 25 min after injection. The pattern of movement was consistent with bulk interstitial fluid flow influenced by hydrostatic pressure. In osteons sectioned transversely, evidence was seen of centrifugal movement of ferritin in a halo pattern around the central capillary. Haloes in the superficial part of the cortex coalesced to form a ferritin front which moved toward the periosteal surface. Movement of ferritin was delayed by appositional bone of the periosteal surface. Ferritin was seen in channels in the matrix and perivascular spaces termed "matrix prelymphatics" and "perivascular prelymphatics," respectively, because they lacked an endothelial lining.