The ramus interventricularis ventralis (RIV) of the dog heart was perfused with donor blood from the femoral artery. The inflow and outflow pressure, the blood flow and the diameter of the RIV were recorded. An increase in the blood flow from 11.3 +/- 0.3 to 81.4 +/- 6.2 ml/min induced, in 24.4 +/- 1.7 s a gradual increase in RIV diameter which became stabilized in 90-120 s and amounted to 80.87 +/- 11.68 microns, i.e. to 3.37 +/- 0.70% of the resting diameter. Since the mean pressure at the site where the diameter was recorded was maintained constant, relaxation of RIV smooth muscle has been assumed to have occurred. Dilation could not be evoked after RIV had been dilated by papaverine. Any interference by changes in pressure amplitude accompanying the increase in blood flow was precluded by using non-pulsating perfusion of the RIV. An increase in the non-pulsating blood flow from 12.11 +/- 2.20 to 90.33 +/- 11.30 ml/min likewise--in 19.50 +/- 1.83 s--produced an increase in RIV diameter of 1.8%. Blood flow is being suggested as one factor regulating the tone of the smooth muscle of the major coronary artery. This mechanism is supposed to counteract sympathetic constriction of the major coronary artery.