ABSTRACTWe studied the effects of the pericardium on diastolic left coronary pressure-flow relationships in heart-blocked and isolated canine preparations. In these preparations, the left and right coronary arteries were dilated with adenosine and perfused by means of a pressurized arterial reservoir. The diastolic left heart pressure (LHP) was controlled by the height of a reservoir connected to the left atrium and left ventricle. The right atrial and ventricular pressure i.e., coronary outflow pressure, was kept constant at 0 mm Hg. Before and after pericardiectomy, diastolic coronary pressure-flow relationships were obtained at three values of LHP (0, 15, and 30 mm Hg) with driving pressure decreasing (2 mm Hg/sec or less) from approximately 60 mm Hg to the actual zero-flow pressure (Pf = 0) during a single long diastole induced by cessation of ventricular pacing. The slopes of the coronary pressureflow relationships were approximated by a linear regression analysis in which the correlation coefficients were greater than .98 in all cases. Before pericardiectomy, with LHP increasing from 0 to 15 and 30 mm Hg, the value of Pf = 0 significantly increased from 7 + 1 to 16 ± 1 (p > .01) and 28 + 2 mm Hg (p > .01 ), respectively. After pericardiectomy, it increased from 7 + 1 to 14 + 1 (p > .01 ) and 17 ± 2 mm Hg (p > .01), respectively. When LHP was at 0 and 15 mm Hg, the pericardiectomy had no effect on the value of Pf = 0. However, pericardiectomy lowered the value of Pf = 0 (p > .01) when LHP was at 30 mm Hg. On the other hand, the slopes of the regression lines were not significantly changed by either an increase in LHP or by pericardiectomy. From these results, we conclude that the pericardium plays an important role in regulating the coronary circulation by changing the effective back pressure, i.e., Pf = 0, when the intracardiac pressure increases beyond a certain degree.