Heat related injuries pose a significant health threat to many occupational workers, recreational participants and endurance athletes. Elevated body temperatures may lead to heat-related illness, dehydration, compromised cardiovascular function and impaired cognitive function. However, practitioners and clinicians continue to struggle to find appropriate methods to obtain accurate temperature measurements outside of a laboratory setting. Fire fighting involves strenuous physical activity in a hot environment while wearing protective gear and thus provides a useful model to compare body temperatures under hyperthermic conditions. The purpose of this study was to compare the body temperature measured at different sites before, immediately after, and 15 minutes after fire fighting activity. The body temperature of nineteen (n=19) apparently healthy fire fighting recruits was measured before fire fighting, immediately after fire fighting, and after approximately 15 minutes of recovery. Participants engaged in approximately 15 minutes of fire fighting activity (conducting a primary search, advancing a charged line, and extinguishing a fire) while inside a training structure that contained live fires. Body temperature was measured at three sites: the oral cavity (via an electronic thermometer), the tympanic membrane (via an infra-red tympanic thermometer), and the rectal cavity (via an electronic thermometer). The data reveal that baseline tympanic temperatures (34.0+/- 1.0 C) are lower than oral temperatures (36.4+/- 0.3 C) and that oral temperatures are lower than rectal temperatures (37.1 +/- 0.5 C). Body temperature, as assessed at all three sites, increased significantly as a result of fire fighting activities. However, the increase measured at the tympanic site (1.5 C) was greater than the elevation measured at the oral (0.4 C) or rectal (0.8 C) sites. Also, only tympanic temperature decreased significantly from immediately post fire fighting to 15 minutes post fire fighting. In summary, it is important to be able to assess the body temperature of fire fighters (and athletes) accurately and conveniently. Unfortunately, there are significant limitations to the sites that are commonly assessed. The oral cavity, an easily assessed site does not provide an accurate assessment of temperature following fluid ingestion and therefore may provide erroneous information if the participant has ingested fluid before the measurement is taken. Rectal temperatures are generally considered accurate, but are inconvenient. Tympanic sites are convenient but the accuracy of this site has not been determined under fire fighting conditions or in high ambient temperatures. Given the importance of accurate temperature measurements, additional research is warranted.Keyword(s): exercise/fitness, measurement/evaluation, safety/injury prevention