The aim of the study was to investigate the physiological meaning of the positive peak which appears at the onset of ventricular ejection on traces of blood flow in the left coronary artery. It was proposed that the protosystolic peak could represent systolic charging of epicardial coronary arterial compliance, i.e. the compliance which is not squeezed by myocardial contraction and which resides in superficial coronary arteries. To verify this hypothesis, blood flow was recorded from the left circumflex coronary artery in five anesthetized open-chest dogs and the protosystolic peak was identified by visual analysis or on the basis of zero-crossing of the first derivative. An index of epicardial compliance (delta V/delta P) was derived by dividing the peak area (delta V) by the aortic pulse pressure (delta P). Under basal conditions, the estimate of epicardial compliance, amounting to 0.271 +/- 0.149 x 10(-3) ml/mmHg (2.04 +/- 1.12 x 10(-12) m4s2kg-1; mean +/- SD), fell in the lower part of the range of values found by different authors and increased during hemorrhagic hypotension, due to nonlinearities of compliance in general. Similar values of epicardial compliance were obtained when a lumped resistance-capacitance parallel model was fitted to systolic coronary blood flow. Unexpectedly, however, the protosystolic peak was greatly decreased during coronary reactive hyperemia. We conclude that the protosystolic peak can be used as an index of epicardial compliance, but only at basal coronary vasomotor tone.