The fractions of left anterior descending (LAD) and circumflex (LC) inflow drainage into the canine great cardiac vein (GCV) and coronary sinus (CS) have been quantitated by use of a right heart bypass preparation in which GCV outflow was isolated from the remainder of CS outflow. Following direct LAD injection of indocyanine green dye (ICG), 63 +/- 8% (SD) of the total amount of dye recovered appeared in GCV outflow and the remainder in CS outflow. CS recovery of ICG was decreased appreciably by ligation of epicardial venous connections between the LAD and LC beds, but was not affected by selective reductions of LAD or LC inflow. Only 3 +/- 3% of ICG injected into the LC was recovered in GVC outflow under basal conditions, and these low values were not affected measurably by selective reductions of LAD or LC inflow. CS drainage of LAD inflow could be augmented by selective increments of GCV pressure exceeding 7-10 mmHg. Increments of LC drainage in GCV outflow required CS pressures that exceeded GCV pressures by greater than 10 mmHg.