OBJECTIVE: The inducible isoform of nitric oxide synthase (iNOS) is known to be a trigger of the heat stress (HS)-induced cardioprotection. Since iNOS also appears to mediate various forms of myocardial preconditioning, the goal of this study was to investigate its role as a mediator of the HS response. METHODS AND RESULTS: Male Wistar rats were divided in six groups, subjected or not to HS (42 degrees C internal temperature, for 15 min). Twenty-four hours later, they were treated or not with either L-NAME, a non-selective inhibitor of NO synthase isoforms, or 1400W, a selective iNOS inhibitor, 10 min before being subjected to a 30-min left coronary artery occlusion followed by a 120-min reperfusion, in vivo. The infarct size (tetrazolium staining) reducing effect conferred by heat stress (from 46.0+/-1.4% in sham to 26.8+/-3.8% in HS groups) was completely abolished by both L-NAME (53.9+/-3.1%) and 1400W (51.8+/-3.3%). Additional studies using Western blot analysis demonstrated a 3.8-fold increase in myocardial iNOS protein expression 24 h after HS. CONCLUSION: These results suggest an involvement of iNOS as a mediator of the protection conferred by heat stress against myocardial ischaemia.