Prior heat stress leads to an enhancement of postischemic mechanical function and improvement in biochemical indices of injury in the rat heart, associated with an elevation in endogenous catalase activity. We have examined the effect of heat stress on free radical release during reperfusion in the isolated rat heart using electron spin resonance (ESR). Twenty four hours after heat stress or sham treatment, hearts were perfused in the Langendorff mode and subjected to 10 min of no-flow global ischemia followed by 10 min of reperfusion. Coronary effluent was collected at specific time points in PBN for ESR measurement. A PBN adduct was identified with characteristics consistent with an alkoxyl radical: PBN-LO. In sham hearts there was a rapid rise in adduct release to a maximum (228 +/- 15% of stabilization values, p < .05) occurring 1 min into reperfusion. In heat stress hearts there was no significant rise in adduct release during the reperfusion period. Pretreatment of hearts with 3-amino triazole, an inhibitor of catalase, failed to clarify whether the protection seen in heat stress hearts was a result of the elevation in catalase activity. These results suggest that heat stress protects the myocardium against the oxidative stress of ischemia-reperfusion.