Intense exercise leads to the accumulation of the inducible member of the 70 kDa family of heat-shock proteins, Hsp70, in male, but not female, hearts. Estrogen is at least partially responsible for this difference. Since androgen receptors are expressed in the heart and castration leads to decreases in calcium regulatory proteins and altered cardiac function, testosterone or its metabolites could also be involved. We hypothesized that removal of endogenous testosterone (T) production through castration would reduce the accumulation of cardiac Hsp70 after an acute exercise bout while castrated animals supplemented with 5-dihydrotestosterone (DHT) would show the intact male response. Fifty-four, male, 8 week old Sprague-Dawley rats were divided into intact, castrated or castrated supplemented with DHT groups (n=18/group). At 11 weeks of age, 12 of the animals in each group undertook a 60 min bout of treadmill running at 30 m/min (2% incline) while the remaining 6 in each group remained sedentary. At either 30 min or 24 hours post-exercise (n=6/timepoint), blood and hearts were harvested for analysis. Serum T was undetectable in castrated and DHT-treated, castrated rats, whereas serum DHT was significantly reduced in castrated animals only (~60% reduction) (p<0.05). Although there were no differences in constitutive levels of Hsp70 protein, exercise significantly increased cardiac Hsp70 mRNA and protein in intact and DHT-supplemented rats, but not in castrated animals (p<0.05). To examine whether castration eliminated the ability to respond to stress, an additional 6 intact and 6 castrated animals were subjected to a 15 min period of hyperthermia (core temp raised to 42°C) and sacrificed 24 h later. As opposed to exercise, castrated animals subjected to heat-shock exhibited increases in Hsp70 above non-shocked (i.e. sedentary) animals similarly to intact males (p<0.05). These data suggest that androgens, in addition to estrogen, play a role in the sexual dimorphism observed in the stress response to exercise but not heat-shock.