Purpose : To investigate changes in the size and degree of signal enhancement of reperfused myocardium duringthe subacute stage of an ischemic episode, using Gd-DTPA-polylysine enhanced magnetic resonance imaging. Materialsand Methods : In six cats, the left anterior descending artery was occluded for 150 minutes, and this was followedby reperfusion. Contrast enhanced T1-weighted spin echo magnetic resonance imaging using gadoliniumdiethylenetriaminepentaacetic acid-polylysine (Gd-DTPA-polylysine) was performed on the 1st , 2nd, and 6th days ofthe reperfusion period. The size of ischemic myocardium was estimated each day on MR images by measuring the sizeof signal enhanced area and the degree of signal enhancement according to time was measured. After sacrificing theanimals on day 6, the myocardial specimen was histochemically stained with 2,3,5-triphenyltetrazoliumchloride(TTC). Results : Signal enhancement and the size of the ischemic myocardium, as seen on MR images,decreased linearly during the six days of the subacute stage. On the 6th day, however, signal intensity was stillhigher than that of normal myocardium, and the size of signal enhanced area measured on MR images wassignificantly larger than on TTC-stained specimens (p<0.001). Conclusion : We conclude that the size of enhancedarea and degree of signal enhancement decreased linearly during the subacute stage of reperfused myocardialinfarction and that the area of MR signal enhancement during the acute stage includes both irreversibly andreversibly damaged myocardium.