This study was designed to measure early changes in myocardial perfusion after acute coronary occlusion, and to examine the relationships among blood flow, myocardial edema, and magnetic relaxation times of ex vivo myocardial tissue. In ten dogs, the left anterior descending coronary artery was occluded for 4 hours prior to sacrifice of the animals. Regional myocardial blood flow was measured using radiolabeled microspheres (15 micron), which were injected into the left atrium 5 minutes prior to sacrifice. Multiple subendocardial tissue samples from the left ventricular free wall were obtained for measurement of magnetic relaxation times, percent water content and tissue radioactivity. Mild, moderate, and severe ischemia were defined as reductions in myocardial blood flow to 30% to 50%, 15% to 30%, and less than or equal to 15% of control, respectively. Myocardial water content was increased with mild ischemia (79.6 +/- 0.7%), moderate ischemia (79.9 +/- 0.4%), and severe ischemia (80.3 +/- 0.6%), all P less than .005 vs. control. T1 relaxation times rose with mild (544 +/- 10 msec, P less than .005 vs. control), moderate (543 +/- 11 msec, P less than .005 vs. control), and severe ischemia (574 +/- 10 msec, P less than .001 vs. control). T2 relaxation times behaved in a similar manner, being prolonged in the mildly, moderately, and severely ischemic groups (38.3 +/- 0.3, 38.1 +/- 0.3 and 38.2 +/- 0.3 msec, respectively; all P less than .001 vs. control).(ABSTRACT TRUNCATED AT 250 WORDS)