This study was undertaken to evaluate the coronary collateral circulation of the goat. Year old, castrated male goats were anesthetized with sodium pentobarbital. A branch of the left circumflex coronary artery was dissected and a snare placed around it. Regional myocardial blood flow was measured by injecting colored microspheres into the left atrium before and during 3 hr of occlusion of this coronary artery. The Area At Risk for infarction, as defined by a left atrial injection of Brilliant Green dye, was divided into a central Ischemic zone and a peripheral Ischemic Border zone. The degree of overlap in the blood flow distributions between the risk and the nonrisk areas was quantitatively assessed by injecting microspheres directly into the artery to the Area At Risk. Baseline blood flow to the normally perfused goat myocardium was 1.13 +/- 0.18 ml/min/g (mean +/- SE). Following occlusion, the flow to the Ischemic zone was 0.07 +/- 0.03 ml/min/g and to the Ischemic Border zone was 0.31 +/- 0.18 ml/min/g. Flow to either zone did not increase during the 3-hr observation period. Overlap at the perimeter of the risk and nonrisk areas was approximately 22%. We conclude that flow to the Ischemic zone is low because the goat has few native collateral blood vessels, and that flow to the Ischemic Border zone is significantly affected by overlap with normal myocardium.