We compared the accuracy of an ingestible telemetry pill method of core temperature (Tc) measurement and an infrared tympanic membrane thermometer to values from a rectal thermistor during exercise-induced heat stress. Ten well-trained subjects completed four exercise trials consisting of 40min constant-load exercise at 63% of maximum work rate followed by a 16.1km time trial at 30°C and 70% relative humidity. Temperature at rest was not different between the three methods ofTcmeasurement (Tre: 37.2±0.3°C;Tp: 37.2±0.2°C;Tty: 37.1±0.3°C;P=0.40). Temperature rose continuously during the exercise period (ΔTre: 2.2±0.5°C; ΔTp: 2.2±0.5°C; ΔTty: 1.9±0.5±°C and there were no differences betweenTreandTpmeasurements at any time throughout exercise (P=0.32). While there were no differences betweenTreandTtyafter 10min (P=0.11) and 20min (P=0.06) of exercise,Ttywas lower thanTreafter 30min of exercise (P<0.01) and remained significantly lower throughout the remainder of the exercise period. These results demonstrate that the telemetry pill system provides a valid measurement of trunk temperature during rest and exercise-induced thermal strain.Ttywas significantly lower thanTrewhen temperature exceeded 37.5°C. However, whether these differences are due to selective brain cooling or imperfections in the tympanic membrane thermometer methodology remains to be determined.