The pattern of the cortical vascular network and the changes occurring in it during the formation of Haversian systems have been studied in the diaphyses of the tibiae of dogs of various ages. The number, area, perimeter and microscopic structure of the vessels have been investigated with respect to the size of the Haversian canals and length of the osteoblastic layer, using India ink perfusion of the vascular network. The vascular network in compact bone undergoes conspicuous changes as a result of bone growth and turnover processes. The vessels proliferate during the formation of resorption cavities, eventually forming small plexuses of capillaries and tortuous venules. As osteons are laid down and their canals become more narrow, the progressive disappearance of the largest vessels leads to a reduction in the number of vessels in each canal. Morphometric analysis reveals a close correlation between the rate of osteonic bone deposition, the size of osteoblasts and the surface area of the vessels.