The traditional use of core temperature to assess the thermal effects of clothing has recently been questioned. The purpose of this study was to assess the reproducibility of body temperature in five subjects (mean age, 22.6 +/- 1.5 years) wearing either athletic clothing or a chemical protective overgarment while exercising at 20 degrees C and at 40 degrees C. The exercise was preceded by a 1 h adaptation period in a controlled environmental chamber. Results indicated that mean group change in rectal temperature (delta Tr) appeared to be reproducible for both garment ensembles at 20 degrees C but not at 40 degrees C. For mean change in oesophageal temperature (delta T(oes)) at 20 degrees C, reproducibility was obtained for the overgarment but not for the athletic garment; at 40 degrees C, mean delta T(oes) appeared to be reproducible with both garments. However, when individual responses were examined, there was little reproducibility for either delta Tr or delta T(oes). In addition, these measurements failed to show differences in the types of clothing worn. It was concluded that the use of core temperature to assess heat stress imposed by wearing clothing during exercise may lead to erroneous conclusions.