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The effects of transient ischemia and reperfusion on regional myocardial function, salvage and swelling have been systematically analyzed in experimental canine preparations. The results of these interventions on myocardial in vitro measurements of magnetic relaxation times (T1 = magnetization recovery, T2 = spin echo) are of significant importance with respect to future nuclear magnetic resonance tomographic imaging. Thus, using a pulsed magnetic resonance spectrometer (10.7 MHz), myocardial tissue samples from two groups of dogs were evaluated. In group 1 (n = six dogs), the left anterior descending artery was occluded for 3 hours before sacrifice; in group 2 (six dogs), 3 hours of occlusion was followed by 1 hour of reperfusion. Multiple tissue samples from normal and ischemic (or ischemic and reperfused) myocardium were obtained for measurement of T1, T2 and % water content (wet weight--dry weight/wet weight). Water content increased with ischemia (78 +/- 4%) and reperfusion (81 +/- 4%) (both p less than 0.01 versus control values). Values for T1 increased with ischemia (598 +/- 39 versus 487 +/- 23 ms in normal tissue from the same heart, p less than 0.01). Even greater T1 changes occurred in the animals with reperfusion (654 +/- 52 ms, p less than 0.01 versus the intra-animal control values). Changes in T2 were similar but less marked (ischemic zone 43.9 +/- 1.0 versus 41.2 +/- 1.0 ms in nonischemic tissue in the corresponding heart, p less than 0.05; reperfusion zone 48.3 +/- 3.5 versus 41.9 +/- 2.3 ms in the normal zone, p less than 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)