&NA;To determine whether selectively altering the coronary perfusion pressure in the adjacent nonoccluded vessel has any influence on the occluded hed, the effects of alterations in the perfusion pressure of the left anterior descending coronary artery on the perfusion and function of the acutely occluded left circumflex coronary (LC) arterial bed were studied in 10 anesthetized open‐chest dogs. Radiolabelled microsphere‐assessed regional myocardial perfusion and endocardial excursion determined by two‐dimensional echocardiography were measured during control conditions prior to mid‐LC occlusion with left anterior descending coronary arterial pressure (LADP) equal to aortic pressure (AoP) (Stage 0) and to 3 randomly performed postocclusion stages. At each postocclusion stage, the perfusion territory of the occluded LC bed (area at risk) was measured in vivo using myocardial contrast two‐dimensional echocardiography. During Stage 1 (LADP = AoP), area at risk was 5.1 ± 0.9 cm2(x± 1 SD) and transmural blood flow to the LC arterial bed decreased from 0.96 ± 0.50 ml/min/g (Stage 0) to 0.16 ± 0.12 ml/min/g (p<0.01), while endocardial excursion decreased from 28.0 ± 9.0% to 2.0 ± 10.0% (p<0.01). During Stage 2 (LADP> AoP), area at risk decreased to 4.4 ± 1.0 cm2compared with Stage 1 (p<0.01), and transmural blood flow, endocardial:epicardial blood flow ratio, and endocardial excursion increased to 0.51 ± 0.39 ml/min/g, 0.64 ± 0.20, and 14 ± 6%, respectively (p<0.01). In contrast, during Stage 3 (LADP < AoP), although the area at risk increased (5.6 ± 0.7 cm2,p<0.01) and transmural blood decreased (0.10 ± 0.10 ml/min/g,p<0.01) compared with Stage 1, endocardial blood flow, endocardial:epicardial blood flow ratio, and endocardial excursion were unchanged (0.11 ± 0.16 ml/min/g, 0.52 ± 0.30, and 1.0 ± 4.0%, respectively). We conclude that significant lateral border zones exit during acute coronary ischemia, which can be influenced positively by selectively increasing the collateral driving pressure. In contrast, although the area at risk increases, when the collateral driving pressure is decreased, the endocardial blood flow and excursion in the area at risk do not further decrease. (Circulation Research1987;61:77‐85)