In the normal heart, transient and adjusted steady state coronary flow rates in response to changes in perfusion pressure are not significantly affected by collateral flow into and from heart muscle surrounding the area perfused by a test artery. An active autoregulatory adjustment tends to counteract flow changes after perfusion pressure is changed. This adjustment is sometimes sufficient to increase vascular resistance above control values. Usually, however, coronary resistance decreases at higher perfusion pressures even though autoregulation is present. The experiments provide no evidence for reflex control of coronary resistance in response to increases in aortic pressure; the initial flow changes can be accounted for by the corresponding increase in perfusion pressure. It is necessary to examine the entire coronary flow pattern after a sudden change in perfusion pressure in order to formulate concepts which include all the important determinants of coronary blood flow.