The authors studied with 99m-Tc Cardiolite (methoxy-isobutyl-isonitrile) SPECT examination the effect of coronary bypass operation on the myocardial perfusion. One week before and 3 weeks after the operation examinations were performed first with dipyridamol loading and 24 hours later in rest. The regional perfusion of the myocardium was studied on the basis of transversal section-pictures of corrected axis position. In the examinations preceding the operation the global sensitivity of the method was found to be 100%. In the demonstration of the structures of the ramus descendens 95%, of the ramus circumflexus and of the right coronary artery 100% sensitivity was reached. In postoperative examinations following the bridging of the ramus descendens anterior the improvement of the perfusion of the corresponding ventricular segment was found in 15/20 during the loading and in 7/8 cases following the bridging of the ramus circumflexus and right coronary artery. Improving perfusion was demonstrated in 17/20 and in 6/8 cases in rest. On the basis of the results the authors consider their method suitable for the study of the regional blood perfusion and for the examination of the changes taking place upon coronary bypass operations.