Oxygen radicals have been implicated in the pathogenesis of myocardial injury. Enhanced chemiluminescence is a sensitive technique for continuous nondestructive measurement of oxygen radical generation. Using an isolated perfused rat heart model, we studied the effect of variable durations of ischemia on oxygen radical generation and postischemic myocardial function. Peak postischemic oxygen radical generation was higher with an intermediate period of ischemia (11.5 min; 528 +/- 53 counts/s) than with either a shorter period (5 min; 328 +/- 21 counts/s) or a prolonged period (40.8 min; 286 +/- 53 counts/s). The magnitude of oxygen radical generation did not correlate with postischemic mechanical function, although it was related to the duration of ischemia with regard to brief and intermediate periods of ischemia (both associated with limited mechanical damage). The increased reperfusion chemiluminescence seen with the intermediate versus the brief ischemic insults can be explained by time-dependent enhancement of the mechanisms present during ischemia that serve to increase oxygen radical generation during reperfusion. In contrast, the longer period of ischemia, resulting in severe mechanical dysfunction, was associated with lower levels of chemiluminescence than observed with an intermediate ischemic duration. This most likely results from the irreversible myocardial injury associated with prolonged ischemia and the consequent inability to generate oxygen radicals. We conclude that, although reperfusion-associated enhancement of myocardial free radical generation may be related to mild to moderate postischemic mechanical dysfunction (stunning), this mechanism may not be of importance in the generation of irreversible reperfusion myocardial injury.