The effects of coronary capacitance on instantaneous pressure-flow (P/F) relationships were analyzed using a theoretical model of coronary flow during diastole that included capacitance. The magnitude of the discrepancy between actual intramural and instantaneously derived P/F relationships was predicted to be dependent on the ratio of two natural decay constants (central aortic decay constant/intrinsic coronary decay constant). The effects of coronary capacitance are eliminated using constant pressure conditions. The instantaneous (dynamic) and constant pressure (static) P/F relationships were compared experientally using a reservoir to provide constant pressure perfusion during prolonged diastoles in heart blocked dogs. In the presence of coronary tone, zero flow pressure intercepts (P(zf)) of 27.1 ± 6.6 and 11.0 ± 3.0 mm Hg were obtained under dynamic and constant pressure conditions respectively, P <0.001. After maximal vasodilation, P(zf) of 14.2 ± 4.5 mmHg and 10.7 ± 2.4 mmHg were obtained under dynamic and constant pressure conditions, respectively, P = NS. P(zf) derived under constant pressure conditions were independent of the state of coronary vasomotor tone with a value about 11 mmHg. The slopes of the dynamic P/F relationships tended to be greater than those derived from constant pressure conditions. This may suggest an additional component of increasing coronary resistance during diastole that could not be readily assessed under dynamic conditions. We conclude that coronary capacitive effects and resistance changes during diastole severely limit the interpretation of instantaneous dynamic P/F relationships. Diastolic coronary perfusion ceases at about 11 mm Hg and is independent of coronary tone when capacitive effects are eliminated.