Studies in cultured cells have demonstrated that non-steroidal anti-inflammatory agents can potentiate heat-induced hsp70 expression through activation of HSF1 to a DNA binding state. We investigated the influence of aspirin on hsp70 expression in intact rats subjected to heat stress. Rats were injected intraperitoneally either with aspirin (100 mg/kg) or vehicle alone, 60 min prior to their placement at 37 degrees C or room temperature for 30 min. hsp70 mRNA expression was analyzed in lung, liver and kidney isolated from animals assigned to one of four different treatment paradigms; untreated controls, heat, aspirin, and aspirin-plus-heat. Comparison of hsp70 expression in the treatment groups revealed that in all tissues examined, aspirin-plus-heat treatment resulted in 3-4 fold higher levels of hsp70 mRNA relative to those seen with heat treatment alone. Little or no hsp70 mRNA expression was detected in the unheated groups, regardless of aspirin treatment. In keeping with the mRNA expression, Hsp70 protein levels were also elevated in aspirin-plus-heat treated animals. Aspirin treatment did not alter hsp70 protein expression in the absence of heat. In contrast to in vitro observations, aspirin treatment in vivo did not alter HSF1 DNA binding properties. Core body temperature measurements revealed that aspirin pretreatment enhanced the rise in body temperature seen in response to heat treatment. This increased hyperthermic response to heat stress probably accounts for the potentiation of hsp70 expression observed in aspirin-plus-heat treated rats. Given the widespread use of aspirin in humans within a dose range comparable to that used here, our findings are likely to have important physiological consequences.