Adult, embryonic, and tumor tissues were grafted to the chorioallantoic membrane of the chick embryo to determine whether blood vessels originally within implants were reused in the establishment of a new blood supply. Grafts were examined daily by in vivo stereomicroscopy and in histologic sections. Colloidal carbon injections into the host vasculature served to confirm the precise onset of graft circulation; Preexisting tumor blood vessels disintegrated by 24 hours after implantation and revascularization occurred at 3 days by penetration of proliferating host vessels into the tumor tissue. Adult tissues did not revascularize, and the original graft vasculature progressively disintegrated during the 9 days of observation, Most embryonic tissues revascularized in 1 or 2 days by reperfusion of the existing graft vasculature. Anastomosis of host and graft blood vessels seemed to result from connections between newly formed vascular sprouts arising from both vasculatures. This study indicates that only tumor grafts acquire their blood supply solely by formation of new blood vessels from the host microvasculature. By contrast revascularization of normal tissues, when it does occur, is predominately the result of perfusion of the preexisting graft blood vessels.