Heparin is known to inhibit vascular myoblast proliferation, migration and matrix synthesis. This study was undertaken to determine (1) whether heparin inhibits (DAIH), (2) DAIH distibution, and (3) the histocytology of DAIH. Thirty-five adult mongrel dogs received aortoiliac dacron bifurcated grafts with heparin impregnation in one limb followed by collagen coating of both limbs. Grafts were explanted at 2, 4, and 6-8 months for determination of DAIH formation, localization, characterization and comparison of graft limb patency. Ten animals died prior to explantation, 25 animals were explanted on schedule. 29 grafts were analysed for DAIH according to the protocol. 18 graft analysed were occluded. The distribution of the patent grafts were: at 2 months 1; at 4 months 3; at 6-8 months 3. DAIH, a multi-interlamination of myoblasts alternating with fibrocollagen occurred specifically at the toe>heel>floor. DAIH plotted against time is summarized below.DAIH is attenuated by heparin only after the superimposed collagen coating is dissolved in 6-8 weeks. The inhibiting effects of heparin on myoblasts ceases after its release in 2-6 weeks. This explains the benefits to the 4-month group mostly in this model. This study supports the tenet that controlling DAIH by controlling myoblast activity using heparin requires uninterrupted treatment. Because flow turbulence, the principal mechanism responsible for DAIH formation is always present in the end-to-side distal anastomosis.