Intraoperative graft flow responses in 15 patients who underwent coronary artery bypass grafting (CABG) were studied systematically. The mean blood flow for 13 left anterior descending (LAD) coronary artery grafts was 64 +/- 20 ml/min and for 12 right coronary artery (RCA) grafts was 53 +/- 13 ml/min. Of these, systolic flow was dominant in one LAD and three RCA grafts, suggesting the perfusion of the right ventricle or the noncontractile left ventricle segment, or both. Five LAD and two RCA grafts had a negligible reactive hyperemic response, and the underlying cause for this can be futher examined by observing their phasic flow patterns. Temporary occlusion of the coronary artery proximal to the site of anastomosis produced increased flow in 8 grafts, indicating the presence of competitive flow; decreased flow was observed in 15 grafts, indicating the presence of proximal retrograde flow which may play a role in the proximal occlusion of the bypassed artery later. Thus intraoperative flow studies supplement the preoperative angiographic findings in elucidating the pathophysiology involved and are useful in evaluating CABG operations.