To prove the effectiveness of the IABP in case of impending myocardial infarction in one series of experiments myocardial blood flow changes due to IABP in dogs with stenosis of the left circumflex coronary artery (LC) were investigated using tracer microspheres. In a control series the left anterior descending artery (LAD) was occluded. In acute LAD occlusion the IABP caused an increase of blood flow in the epicardial layer. In case of LC stenosis there was a far greater increase in blood flow by the IABP most prominent in the endocardial layer. Our experiments indicate that the IABP seems to be effective in impending myocardial infarction.