Thallium 201 is rapidly fixed by the healthy myocardium and its radiation detectable by a scintillation camera. It may be used to reveal and localise ischaemic zones after intravenous injection. In 74 patients undergoing an exercise test, the results of so-called effort scintigraphy were compared with effort electrocardiograms, coronary arteriography and scintigraphy after the injection of Thallium 201 at rest (rest scintigraphy). The results of effort scintigraphy show a high degree of agreement with those of the effort electrocardiogram and of coronary arteriography. It would appear to be of great value in the determination of the permeability of aorto-coronary by-passes.