The mammalian coronary microcirculation is a complex network of anastomosing vessels. Previous studies have documented the presence of anatomoses between capillaries originating from a common artery; however, the existence of anastomotic vessels connecting different arteries remains a subject of controversy. The objective of the current study was to determine the extent of anastomotic connections between the left anterior descending (LAD) and circumflex (Cx) beds at the capillary level in the canine heart. Red phthalo pigment (Dupont: Wilmington, Del.) was perfused in vitro, at 120 mm Hg, into the LAD bed, while blue pigment was perfused simultaneously, under the same pressure, into the Cx. Capillary anastomoses between the LAD and Cx were confirmed histologically by the consistent presence of vessels at the junction of the two perfusion fields filled with both colors of pigment. In normal dog hearts, 21.9 ± 2.8% (mean ± SEM) of vessels at the junction of the LAD and Cx were perfused by both arterial trees; the percentage capillary anastomoses did not vary significantly with location, apex to base, in the hearts. In addition, the proportion of capillary anastomoses was not altered by either 1 or 6 hr of coronary artery occlusion followed by reperfusion (21.3 ± 2.8% and 22.5 ± 2.6%, respectively), or by postmortem cautery of the epicardial collateral vessels (21.8 ± 5.1%). To assess the potential functional role of these capillary interconnections, regional blood flow at the outer margin of the LAD bed, measured during LAD occlusion by injection of radioactive microspheres, was considered as a function of percentage capillary anastomoses. There was a positive correlation (r = 0.87) between the proportion of anastomoses between the two circulations and blood flow at the outermost edge of the LAD bed, while the LAD was occluded. These results suggest that the anastomoses between the LAD and Cx beds in canine hearts could be important in maintaining blood flow at the extreme outer edge of the perfusion field, if flow through one of the arteries is impaired.