Sprague Dawley rats were either microwave exposed (2 GHz) or subjected to overheating by exercise on a treadmill at room temperature or in a warm room. Initial studies showed tympanic temperatures of 43oC were necessary to induce hsp70 protein expression in the brain regardless of heating rate. In studies using temperature probes inserted into the hypothalamus of these rats, hypothalamic temperatures of 41.9-42.2 were necessary to cause work stoppage (treadmill exercise). Interestingly, however, microwave pretreatment resulted in a different pattern of brain hsp70 expression than exercise in a warm room, and exercise alone was not sufficient to induce brain hsp70. Exercise alone was, however sufficient to induce hsp70 in skeletal and cardiac muscle as well as a variety of other tissues, but it was unclear whether these other tissues experienced temperature increases, (perhaps the rats simply pooped out for other reasons prior to a significant brain temperature elevation with exercise alone). With microwave exposures, hsp70 induction was greatest in the ventral & hindbrain regions. With exercise + warm room induced temperature elevations, hsp70 induction was more homogeneous in forebrain, midbrain, and hindbrain. Temperature probes inserted into the hypothalamic and cortical regions of conscience rats indicated a more rapid increase and higher (2-2.5 fold) final mean temperature elevation in the hypothalamus than in nearby cortex only mm away with microwave exposures, suggesting microwave heating produces non-uniform temperature elevations (or, alternatively, different regional stress responses) in the rat brain. While young and old rats showed different patterns of Hsp70 protein accumulation following the high power RF exposures, the general levels of Hsp70 protein following RF were about the same. The hypothalamic temperatures reported in this study for rat work stoppage (41.9-42.2) correlate well with values reported by Sharma and Hoopes (2003) of core temperature elevations in humans of 41.6 to 42 oC for up to 8 hours leading to brain pathology.