Various sizes of veins and internal thoracic arteries were grafted into unobstructed circumflex coronary arteries in 28 dogs. Grafts did not increase total flow to the distal bed. Flow percentage originating from the graft was compared to the ratio of cross-sectional areas of graft to coronary artery. Results were compared to a theoretical model of the bypass system. With cross-sectional ratios of 1.2:1 and 1.5:1, the graft captured 70% and 85%, respectively, of distal flow; at 1:1 50% and 0.5:1, only 20%. With the coronary artery temporarily occluded, even 0.5:1 grafts handled the basal flow. However, flows two to four times the basal level were carried fully only by grafts of 1.2:1 or greater; smaller grafts carried less—up to 30% reduction at 0.5:1. Length of graft, as well as shape (tapered or not), is considered in applying ratios.