The purpose of this study was to evaluate the effects of acute and chronic ischemic injury on myocardial spin-lattice relaxation times (T1) in dogs. Ligation of the left anterior descending coronary artery was performed on 23 dogs which were divided into four experimental groups and killed at 3 hours (n = 6), 4 days (n = 6), 21 days (n = 5), and 56 days (n = 6) after coronary occlusion. T1 was measured in vitro with a 2.5 kg nuclear magnetic resonance (NMR) spectrometer using tissue from ischemic and control areas of the myocardium. Both the 3-hour and 4-day groups showed prolongations in T1 (p less than 0.01) for tissue from the ischemic area. In the 21-day group, two infarcts showed an increase in T1, two showed a decrease in T1, and one showed no significant change. The 56-day old infarcts had a lower mean value for T1 than control myocardium (p less than 0.01). The differences in myocardial water content between control and infarcted myocardium were found to parallel changes in T1 values in each experimental group. Pathologic examination of the myocardial scar from 21- and 56-day-old infarcts revealed extensive fibrosis in the infarcts with lower T1 values and tissue water contents than control myocardium. We conclude that myocardial edema in 3-hour and 4-day-old infarcts results in association with prolongations in T1. Twenty-one-day-old infarcts may be edematous (with increased T1) or fibrotic (with T1 values lower than normal myocardium), while 56-day-old infarcts are fibrotic and have shorter T1 values than normal myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)