In vitro (10 MHz) proton magnetic resonance (MR) was used to study myocardial infarction 30 hours after ligation of the left coronary artery in rats. The location of large anterolateral myocardial wall infarcts was confirmed when the rats were killed. In vitro measurements of control animals (n = 7) showed T1 prolongation in right as compared with left ventricles (559 +/- 41 vs. 522 +/- 35 milliseconds [sd]; P less than .02); this correlated with a higher water content and is believed to be caused by increased extracellular compartment size. Infarcted myocardium (n = 9) had lower T1 relaxation times than control left ventricle anterior walls (480 +/- 37 vs. 522 +/- 35 milliseconds; P less than .01) despite increased water content (78 +/- 1.1% vs. 76.5 +/- 6%; P less than .01) and a mild increase in T2 values. We conclude that the MR characteristics of myocardial infarction in rats differ from those of other models, possibly because of differences in infarct sizes and rates of proteolysis within the damaged tissues.