Collateral blood vessels from skeletal muscle to myocardium might supplement intramyocardial collaterals during periods of acute myocardial ischemia. This study was conducted to verify the existence of such collaterals and to measure their contribution to collateral flow. In 12 male goats, the innate coronary collateral system to a moderate size myocardial risk area was defined with colored microspheres, and a latissimus dorsi pedicle flap was then apposed to the heart. After 3 weeks, skeletal muscle to myocardial collaterals were characterized by (a) creation of vascular casts (three animals); (b) estimation of skeletal muscle to myocardial collateral blood flow (three animals); and, (c) measurement of total collateral blood flow to the risk area (innate plus skeletal muscle to myocardial collateral flow). Under a dissecting microscope the vascular casts revealed direct communications from the skeletal muscle which penetrated deeply into the myocardium. With the coronary artery to the risk area open, the estimated myocardial collateral blood flow derived from the muscle flap was 0.01, 0.02, and 0.04 ml/min. With the coronary artery to the risk area closed, there was no significant increase in total coronary collateral blood flow. Although the quantity of blood flow delivered by skeletal muscle collaterals was small, this study demonstrates that clearly identified collateral blood vessels form between skeletal muscle and myocardium in a cardiomyoplasty model. This raises the possibility that, under conditions more favorable to their development, extramyocardial collaterals from skeletal muscle might be exploited to augment the intramyocardial collateral system.