To examine the effects of caffeine during exercise in fire protective ensemble (FPE), 10 healthy males completed 3 x 10 min bouts of treadmill exercise on two separate days. Sixty minutes prior to exercise either 6 mg/kg of caffeine (CAFF) or dextrose placebo (PLA) capsules were ingested (randomly assigned, double blind). End-exercise gastrointestinal temperature (T(gi)) was higher in CAFF compared to PLA (38.80 +/- 0.08 degrees C vs. 38.43 +/- 0.11 degrees C, p < or = 0.01). Ventilation (V(E)) and tidal volume (V(t)) were also significantly higher in CAFF, which resulted in higher consumption of air from the self-contained breathing apparatus. While perceived exertion in the caffeine condition was decreased (p < or = 0.05) compared to placebo, the higher T(gi) values increased calculated physiological strain index in CAFF (p < or = 0.01). Caffeine appears to alter the physiological and psycho-physical responses to exercise in FPE and may influence factors related to work tolerance in firefighting. These findings are relevant to occupations such as firefighting where workers are encapsulated during exposure to heavy physical work and/or environmental heat. The results indicate that workers may be more susceptible to heat-related fatigue, illness or injury with ingestion of significant amounts of caffeine. To the authors' knowledge this is the first study involving humans and exercise to detect an increase in body temperature with caffeine ingestion.