The effect of an aortocoronary bypass graft on coronary hemodynamics was studied experimentally in dogs and during surgery for aortocoronary bypass grafting in man. In the experimental part of this study it was shown that coronary dilatory reserve is unaffected between 0 and 50% coronary narrowing. In the presence of more severe stenosis the dilatory reserve declines rapidly. Competition of flow between an aortocoronary bypass graft and the stenotic segment is related to the degree of coronary stenosis. In 53 patients graft flow was measured during surgery. The relation between hyperemic response (hyperemic/control flow = HR) after short occlusion of the graft and the poststenotic coronary pressure (poststenotic pressure/aortic pressure times 100 = PSPR) could be described by the function HR = a. exp b PSPR (r = 0,87). When the degree of coronary stenosis exceeded 80% narrowing, poststenotic pressure decreased rapidly and hyperemic response appeared. In patients with comparable LAD stenosis a close correlation was found between basal graft flow and anterior wall motion (r = 0,91). It is concluded that graft function can be evaluated by the relation between HR, PSPR and the degree of coronary stenosis. Left ventricular wall motion is shown to be an additional determinant of graft flow.