In 42 male Sprague-Dawley rats, the distribution of coronary arteries, the geometry of myocardial infarcts (MIs) and the involvement of the left ventricular papillary and trabecular muscles, after left coronary occlusion, were investigated. The septal branch was found to be responsible for the blood supply to the septum and, thus, occlusion of the left coronary artery, which spares the septal branch, results in an infarct that does not include the interventricular wall. Myocardial infarct size was measured by planimetry of histologic sections of serial slices of the left ventricle. There was no difference between the average percent of subepicardial (43% +/- 1%) and subendocardial (44% +/- 3%) infarction when the whole left ventricle was considered. Analysis of individual slices had shown that the infarcts comprised a greater percentage of left ventricular circumference at the apex than at the base in both layers of the myocardium. Toward the apex, more subendocardium was infarcted than subepicardium. The papillary and trabecular muscles are minimally involved in 75% of infarcts.