The effect of collateral circulation on regional myocardial flow and wall motion of left ventricle was studied on 5 anesthetized dogs with a surgically implanted constrictor on the left circumflex coronary artery (LC). The grade of LC stenosis and wall motion of left ventricle (LV) were determined by cineangiography of the coronary artery and LV at the period of acute and chronic occlusion. Regional myocardial flow was determined by tracer microspheres (TM), labeled with four different isotopes, Sr85, Cr51, Sc46, Ce141. The first TM1 was infused after LC stenosis, TM2 during a temporary complete LC occlusion at the period of acute occlusion; TM3 and TM4 in a similar way 3 to 4 weeks after the acute LC stenosis. Three to four weeks after LC stenosis, i.e. chronic period of occlusion, the degree of LC stenosis progressed from 70--80% to 100% occlusion, but collateral flow and collateral vessels to the ischemic LC area were increased together with an improvement of wall motion of the ischemic LC area. The results may support the idea that collaterals may be an effective compensatory mechanism for ischemia. In contrast to an increase of collateral flow to the ischemic LV free wall in all five dogs, an increase to the posterior papillary muscle was found only in two out of five dogs.