A telemetry monitoring system using an ingestible temperature sensor has recently become available commercially, but to our knowledge no published data on exercise applications have been reported. Consequently, core temperature was measured by both ingestible capsule sensors and standard rectal thermistors in six trained subjects (three cyclists, three runners) during 30 to 90 min of progressive cycling or treadmill exercise. Testing was conducted 3-9 h after ingestion of the capsule. The telemetered temperature was lower than the rectal temperature both at rest and during exercise in every subject, with resulting significant (p < 0.005) mean differences (+/- S.D.) of 36.91 (+/- 0.41) vs. 37.50 (+/- 0.21) and 38.01 (+/- 0.33) vs. 38.94 degrees C (+/- 0.24), respectively. The mean temperature difference increased by 58% from rest (0.59 degrees C) to peak exercise (0.93 degrees C). These preliminary findings demonstrate a consistently lower temperature from the capsule sensor located within the GI tract compared to rectal thermistors. Replication of these results and investigation into this disparity are necessary before these ingestible capsules should be used for routine monitoring of core temperature.