Residual blood flow in pigs (n = 8) and dogs (n = 11) was measured by tracer microspheres (85Sr) 1 hour after occlusion of the left anterior descending coronary artery (LAD). Collateral blood flow was distinguished from overlap flow, defined as the blood flow of non-ischemic myocardium interdigitating into the ischemic area, by direct LAD injection of isotope-labelled microspheres (125I) prior to ligation. In the center of the acutely ischemic pig myocardium the residual blood flow, i.e., the myocardial perfusion remaining after LAD occlusion, was 0.01 +/- 0.01 ml/min/g subendocardially and 0.02 +/- 0.01 ml/min/g subepicardially , as estimated with 85Sr-labelled microspheres. These values were significantly lower than the corresponding values for the dog, 0.13 +/- 0.05 ml/min/kg (p less than 0.05) subendocardially and 0.28 +/- 0.08 ml/min/g (p less than 0.01) subepicardially . In the lateral aspects of the ischemic area, calculations of overlap flow were made with the aid of the distribution of the microspheres injected into the LAD. Values of the residual blood flow were normalized and non-ischemic myocardial perfusion was set to 100 units. In subepicardial layers of the myocardium with calculated overlap flows corresponding to 20, 50 or 80 units, respectively, the residual blood flow (overlap flow + collateral flow) actually measured in the pig was 31 +/- 4, 55 +/- 4 and 75 +/- 7 units and in the dog 65 +/- 6, 79 +/- 5 and 91 +/- 2 units. The values for the dog were significantly different from the respective value for the pig (p less than 0.01). In the subendocardial layers the difference between the two species regarding residual blood flow was similar, although the difference was statistically significant only for myocardium with a calculated overlap flow of 80 units. When the calculated overlap flow was subtracted from the measured residual blood flow, the collateral blood flow was found to be extensive in the dog and virtually absent in the pig. When, in the dog, the collateral blood flow across the lateral border of the ischemic area was related to the amount of myocardium it supplies, it was found to be homogeneously distributed. Thus neither subendocardially nor subepicardially could a gradient of collateral blood flow be detected. It is concluded that in the pig the collateral blood flow is almost nil throughout the acutely ischemic myocardium, both in subendocardial and subepicardial layers. In contrast, the dog has an extensive collateral flow. No lateral gradient of this collateral blood flow could, however, be detected.