Effect of coronary sinus (CS) occlusion on pressure-flow relations was studied at paek reactive hyperemia in anesthetized open-chest dogs during vagal arrest. CS was closed by a specially designed balloon cather. The pressure-flow relations were lineal in both the left anterior descending coronary artery (LAD) and the left circumflex coronary artery (LCX). With CS occlusion, the pressure-flow relations did not change in slope, but shifted to the right in both LAD and LCX. The zero-flow pressure intercept (P(F)=(0)) was 22±4mmHg in LAD, and 17±6mmHg in LCX with the CS open. There was no significant difference in The P(F=0) between LAD ahd LCX. The P(F=0) varied directly with changes in CS pressure caused by CS occlusion. Two Variables fitted closer in LCX than LAD. Even a slight elexation in CSP affected P(F=0). Therefore, it is supposed that CSP worke as an extravascular compression rather than as a perfusion pressure.