A procedure in which an isotope of the metal Thallium is injected into the vein of a person who has been doing prescribed, measured exercises for some time. A gamma- scintillation camera is then used to take images of the heart muscle to check blood flow. A smaller than usual uptake of Thallium 201 in any area indicates a decreased blood flow in that area. The test is repeated after a rest of several hours. Blood flow at that time may still be decreased, indicating some abnormality, or it may be adequate, indicating that blood flow is adequate for non-stressful activities, but not sufficient during exertion.