Simultaneous in vivo infusions of two different colored 10 microns microsphere suspensions into the left anterior descending (LAD; red spheres) and left circumflex (LCx; blue spheres) coronary arteries of nine anesthetized dogs identified a specific region of canine myocardium perfused by both arterial branches. Subsequently, the LAD was ligated and a third (green) set of micropheres was infused into the patent LCx artery. Analysis of 40 microns serial sections of tissue revealed interface zones with capillaries perfused by both arteries. The first zone, defined as the Interface Transistion Zone (ITZ) was formed by an intermingling of microvessels supplied by the parent arteries of the adjacent perfusion territories; it separated tissue containing only one or the other colored microspheres. Another zone, defined as the Boundary Watershed Zone was located within the ITZ and had capillaries containing both red and blue microspheres. The width of ITZ was 53377 +/- 817 microns (mean +/- SD), and the width of the BWZ was 3358 +/- 618 microns. Green microspheres, infused into the LCx following coronary occlusion were also found in the ITZ and BWZ. Furthermore, capillaries perfused exclusively by the LAD before occlusion (tissue with red but not blue microspheres) adjacent to the perfusion interface contained green microspheres as well as red/green aggregates, indicating lateral extension of the LCx perfusion territory. This extension of the LCx territory was quantitated by comparing the location at which densities of green microspheres or green/red aggregates decreased abruptly compared to the location of the original ITZ and BWZ boundaries, respectively. Results showed that LAD occlusion caused a 24% expansion of the ITZ and a 48% expansion of the BWZ. In addition, all expansions were significantly greater in subepicardial compared to subendocardial regions (p < 0.001). These results clearly demonstrate the capability of microvascular anastomoses in providing blood flow to the periphery of an ischemic region. Furthermore, the perfusion interface is labile and might be amenable to manipulation.