Coronary sinus pressure (Pcs) elevation shifts the diastolic coronary pressure–flow relation (PFR) of the entire left ventricular myocardium to a higher pressure intercept. This finding suggests thatPcsis one determinant of zero-flow pressure (Fzf) and challenges the existence of a vascular waterfall mechanism in the coronary circulation. To determine whether coronary sinus or tissue pressure is the effective coronary back pressure in different layers of the left ventricular myocardium, the effect of increasingPcswas studied while left ventricular preload was low. PFRs were determined experimentally by graded constriction of the circumflex coronary artery while measuring flow using a flowmeter. Transmural myocardial blood flow distribution was studied (15-μm radioactive spheres) at steady state, during maximal coronary artery vasodilatation at three points on the linear portion of the circumflex PFR both at low and high diastolicPcs(7 ± 3 vs. 22 ± 5 mmHg;p < 0.0001) (1 mmHg = 133.322 Pa). In the uninstrumented anterior wall the blood flow measurements were obtained in triplicate at the twoPcslevels. From low to highPcs, mean aortic (98 ± 23 mmHg) and left atrial (5 ± 3 mmHg) pressure, percent diastolic time (49 ± 7%), percent left ventricular wall thickening (32 ± 4%), and percent myocardial lactate extraction (15 ± 12%) were not significantly changed. IncreasingPcsdid not alter the slope of the PFR; however, thePzf, increased in the subepicardial layer (p < 0.0001), whereas in the subendocardial layerPzfdid not change significantly. Similar slopes andPzfwere observed for the PFR of both total myocardial mass and subepicardial region at low and highPcs. Subendocardial: subepicardial blood flow ratios increased for each set of measurements whenPcswas elevated (p < 0.0001), owing to a reduction of subepicardial blood flow; however, subendocardial blood flow remained unchanged, while starting in the subepicardium toward mid-myocardium blood flow decreased at highPcs. This pattern was similar for the uninstrumented anterior wall as well as in the posterior wall. Thus asPcsincreases it becomes the effective coronary back pressure with decreasing magnitude from the subepicardium toward the subendocardium of the left ventricle. Assuming that elevatingPcsresults in transmural elevation in coronary venous pressure, these findings support the hypothesis of a differential intramyocardial waterfall mechanism with greater subendo- than subepi- cardial tissue pressure.