It has been demonstrated that temporary occlusion of major epicardial arteries of the dogs produces a nontransmural myocardial infarction (MI) whose size is reduced by early reperfusion. This study was undertaken to determine the location and extent of MI following acute occlusion and reperfusion of the septal artery (SA). The SA was occluded for four hours in group I (7 dogs). Occlusion time for group II (6 dogs) was 2 hours and for group III (6 dogs) was 1 hour, followed by 2 and 3 hours of reperfusion, respectively. The hearts were then removed and cut into transverse slices from base to apex. The triphenyl tetrazolium chloride technique identified the areas of infarction, which were quantitated with a microcomputer-based graphics system. To determine the extent of necrosis across the interventricular septum (IVS), the IVS was divided into 5 transverse segments of equal depth and the amount of MI was determined for each. In group I, MI involved 3.42 +/- 0.9% (mean +/- SEM) of the left ventricle (LV) and 13.49 +/- 3.4% of the IVS. In group II, 6.11 +/- 1.3% of the LV and 25.00 +/- 5.5% of the IVS were infarcted. In group III, 5.63 +/- 1.3% of the LV and 31.9 +/- 14.3% of the septum were infarcted. MI was larger on the left side of the IVS than on the right in all groups, and the extent of MI did not differ significantly between the three groups. This study showed that early reperfusion of the SA did not reduce MI as reported for other coronary beds.