The distribution of intravascularly administered ferritin was examined in histological sections of femurs from 2-day-old chicks. The ferritin was seen in the mineralized portion of the bone as well as in the vessels 5 minutes after injection into the external jugular vein but was essentially absent 4 hours after injection. The concentration of ferritin in the vessels appeared to increase from the endosteal to the periosteal surface. In the mineral the ferritin appeared to move as a front roughly parallel and distal to the canals. A scheme for the movement of fluid and associated material through the mineralized portion of chick bone is presented that suggests that the driving force for the bone fluid is attributable to pressure differentials between adjacent canals. This differential is due to the geometry of the canals and increases as the angle of the canal relative to the endosteal surface decreases.