Endothelium-derived relaxing factors (EDRFs) mediate vasodilation of small arterioles in skeletal muscle under various (patho)physiological conditions: Escherichia coli sepsis, systemic hypoxia, and topical acetylcholine (ACH) application. To test if heat shock changes EDRF-dependent reactivity of arterioles to ACH, we used closed-circuit videomicroscopy in the in vivo cremaster muscle of rats whose systemic temperatures had been slowly raised to and maintained at 41 degrees C. We also tested for ACH responses after increasing cremaster muscle temperatures and maintaining those at 40 degrees C. The experiments showed that EDRF-dependent vasodilation of small arterioles to acetylcholine was substantially attenuated in response to systemic and local heat treatment. In two other animal groups, concentration-dependent vasodilation of small arterioles to sodium-nitroprusside was not as much attenuated in the response to local tissue temperature elevation. This suggests that locally elevated tissue or systemically elevated body temperatures can change generation or efficacy of EDRFs in the post-hyperthermia phase in the skeletal muscle microcirculation.