To investigate the localization of earliest damage in ischemic and ischemic/reperfused myocardium, anesthetized rats were subjected to coronary occlusion for 15, 30, 45, or 90 min. One half of the animals in each group had no reperfusion, while the other half was reperfused for 14 min. Using histological methods, preferentially in the periphery of the area at risk, localized zones were detected which were lacking the hypoxia-specific increase in NADH fluorescence. The extent of these areas displaying injured tissue was found to be significantly smaller in the ischemic non-reperfused hearts than in the ischemic reperfused organs (ischemia 15 min.: 0.22 ± 0.12 % vs. 43.0 ± 5.0 %; 30 min: 5.7 ± 2.7 % vs. 64.6 ± 2.9 %; 45 min.: 5.6 ± 1.2 % vs. 66.0 ± 7.5 %; and 90 min: 39.3 ± 5.5 % vs. 86.7 ± 1.8 % of the area at risk). The results point to a localized initiation of the damage close to the surrounding oxygen-supplied tissue during ischemia and an expansion of this injury by intercellular actions into yet intact areas upon reperfusion.