The contractile force of the deep and superficial myocardial fibers was examined in the open-chest anesthetized dog as a function of coronary blood flow (CBF). When 1) dogs that failed to demonstrate coronary autoregulation were eliminated from the data base and 2) CBF and contractile force data were both normalized as a percent of their vaues when perfusion was from aortic pressure (autoperfusion), the relationship between them became very reproducible. Contractile force was highly dependent on the flow rate when the CBF was below that chosen by autoregulation (the rate during autoperfusion). Conversely contractile force was relatively independent of flow at higher CBF. The contractile force-CBF curve thus was found to break precisely at the autoperfused CBF. When myocardial metabolism was elevated by paired electrical stimulation this relationship was unchanged. It was concluded that coronary blood flow is tightly regulated to match metabolic needs over a range of metabolic rates.