To characterize the mechanisms for myocardial ischemia induced by hemorrhagic shock, 29 dogs were subjected to hemorrhage at a mean aortic pressure (MAoP) of 30-60 mmHg. After 10 min of hemorrhage, the beating hearts were rapidly cross sectioned and freeze clamped to visualize the two-dimensional distribution of myocardial ischemia with NADH fluorescence (NADH-F) in 22 dogs. NADH-F was developed at an MAoP of 40 mmHg or less and involved both the subendocardial half and the subepicardial half of the left ventricle [34 +/- 14 vs. 20 +/- 14% (P < 0.05) and 65 +/- 16 vs. 52 +/- 15% (not significant) of the cross-sectional area of the left ventricular slice at MAoP levels of 40 and 30 mmHg, respectively]. Magnified NADH-F photography demonstrated heterogeneously distributed microischemic lesions with a columnar shape (mode of short-axis length, 60-80 microns). NADH-F-guided microsamplings revealed higher NADH and lactate concentrations in a positive NADH-F area than those in a negative NADH-F area. The ratio of endocardial to epicardial blood flow was maintained at a relatively high level (1.07 +/- 0.07 and 0.88 +/- 0.07 at MAoP levels of 40 and 30 mmHg, respectively; n = 7 dogs), and the reactive hyperemia was preserved as well. In conclusion, myocardial ischemia in early hemorrhagic shock was characterized by minimal transmural heterogeneity and marked heterogeneity between contiguous small regions.