Proton (hydrogen-1) magnetic resonance imaging techniques have potential for the detection and characterization of changes associated with myocardial ischemia. Since image contrast is dependent on T1 and T2 relaxation times, we examined these parameters in a canine preparation of occlusion of the left anterior descending coronary artery. Of 16 dogs studied, seven underwent 3 hr of coronary artery occlusion and nine underwent 3 hr of occlusion followed by 1 hr of reperfusion. After the dogs were killed, the hearts of four from each group were imaged in a small bore, 1.4 tesla magnet. From all hearts myocardial segments were obtained from the normal zone and the central ischemic zone (CZ). These segments were divided into epicardial and endocardial sections and studied in a spectrometer (20 MHz). After 3 hr of occlusion, CZ endocardial T1 and T2 increased significantly (p less than .01 and p less than .05, respectively). Changes in CZ epicardial relaxation times were not as marked. Although T1 and T2 tended to be higher in the reperfused group compared with the nonreperfused group, the differences did not reach statistical significance. In keeping with the spectrometric findings, T1-dependent inversion recovery images and T2-dependent spin-echo images of the excised hearts demonstrated excellent contrast between normal and ischemic myocardium. Modest correlations were noted for both groups between blood flows during occlusion measured by the microsphere technique and T1 and T2 relaxation times. In summary, relaxation times T1 and T2 increase regionally after 3 hr of coronary artery occlusion and tend to be accentuated by reperfusion.(ABSTRACT TRUNCATED AT 250 WORDS)