The possibility will be investigated that passive or active vasomotion of a large coronary artery can convert a partial narrowing to a critical stenosis or complete occlusion. This will be investigated using a non-circumferential stenosis which will be created in such a way that the artery in the area of the stenosis will retain its capacity for active and passive vasomotion. Dogs and pigs with a surgically induced non-circumferential stenosis alone and with circumferential stenosis alone and with circumferential and non-circumferential stenoses in series will be studied. Changes in resistance, caliber and flow in the large coronary arteries with stenoses will be studied in response to pharmacologic and physiologic interventions, including exercise. Hemodynamic methods will be used in open chest animals, and coronary arteriography and radioactive microsphere methods will be used in closed chest animals.