We have shown that isolated blood-perfused heat-stressed hearts are protected only when the blood donor animal has not been exposed to hyperthermia. Systematic hyperthermia results in larger infarction of both isolated control and heat-stressed hearts. In this study we investigated whether indomethacin inhibits in vivo the detrimental effect of hyperthermia. Male rabbits were divided into four groups, that is A(30), B(30), C(30), and D(30), representing hearts that ultimately received 30 minutes of ischemia. In a second series, rabbits were divided into groups A(45), B(45), (C45), and D(45) representing hearts that ultimately received 45 minutes of ischemia, and in a third series were divided into groups A(HSP), B(HSP), C(HSP), and D(HSP) representing animals that were heat shocked and their hearts were used to measure heat shock proteins. All the A groups (heat shocked) were subjected to 42 degrees C hyperthermia, all the B groups to the same procedure but with the addition of indomethacin (heat shocked + indomethacin), all the C groups served as controls, and all the D groups were treated with indomethacin only (control + indomethacin). Twenty-four hours later, all (30) and (45) groups were subjected to ischemia, whereas hearts from all (HSP) groups were harvested for heat shock protein measurements. When the animals were exposed to 30-minute ischemia, a significant difference in the infarcted to risk zone ratio (%I/R) was observed: A(30): 33.0 +/- 5.2, B(30): 16.1 +/- 4.4 [conferring a 51.2% reduction in infarct size, P < 0.05], C(30): 48.9 +/- 4.0, and D(30): 47.8 +/- 3.8 [P < 0.001 vs. B (30) and P < 0.05 vs. A(30)]. However, the %I/R did not differ among any of the (45) groups. Heat shock proteins themselves were seen to increase in A(HSP) and B(HSP) groups. Indomethacin enhances the beneficial effect of heat shock after 30-minute ischemia in vivo, reducing the infarct size by 51.2% in comparison with heat shock.