The changes in spatio-temporal patterns of magnetocardiograms were investigated following injection of superparamagnetic iron-oxide (SPIO) nanoparticles using a dog model of ischemia/reperfusion. Acute myocardial infarction was induced by ligation of the left anterior descending coronary in two anesthetized open-chest dogs. Following 60 min coronary occlusion and 30 min reperfusion, dogs were subjected to injections of SPIO at a dose of 0.56 mg Fe/kg using a catheter inserted into the left atrium. Magnetocardiograms were measured before coronary occlusion, 15 min after reperfusion and immediately after administration of the SPIO. Magnetic field maps of early reperfused myocardium showed spatio-temporal field distributions consistent with anterior myocardial infarction. Magnetic field distribution measured after injection of SPIO revealed additional spatio-temporal features most prominent during ventricular repolarization due to augmentation/fragmentation of the ST-segment detected at several sensor locations. No significant differences in MCG patterns were noted following contrast agent injections in a dog without coronary occlusion. In conclusion, preliminary experimental evidence appears to support the notion that superparamagnetic contrast agents increase the sensitivity of standard MCG and may have an important implication for magnetocardiography in the assessment of regional myocardial ischemia, infarction and perfusion.