The degree of myocardial oxygen delivery necessary to re-establish functional contractile activity after short-term global ischemia in the heart is not known. To determine the relationship of oxygen delivery and recovery of contractile and metabolic function, we used tissue NADH fluorometric changes to characterize adequacy of reperfusion flow. Isolated perfused rat hearts were subjected to global ischemia and reperfused at variable flow rates ranging from 1-100% of baseline flow. Myocardial function and tissue NADH changes were continuously measured. NADH fluorescence rapidly increased and plateaued during ischemia. A strong inverse logarithmic correlation between NADH fluorescence and reperfusion oxygen delivery (DO₂) was demonstrated (R = -0.952). LV function (rate-pressure product) was inversely related to NADH fluorescence at reperfusion flows from 25-100% of baseline (R=-0.922) but not at lower reperfusion flow levels. An apparent reperfusion threshold of 25% baseline DO₂ was necessary to resume contractile function. At very low reperfusion flows (1% of baseline), another threshold flow was identified at which NADH levels increased beyond that observed during global ischemia (3.4% ± 3.0% mean ± SEM, n=9), suggesting further reduction of the cellular redox state. This NADH increase at 1% baseline reperfusion flow was blocked by removing glucose from the perfusate. NADH fluorescence is a sensitive indicator of myocardial cellular oxygen utilization over a wide range of reperfusion DO₂. Although oxygen is utilized at very low flow, as indicated by changes in NADH, a critical threshold of approximately 25% of baseline DO₂ is necessary to restore contractile function following short-term global ischemia.