We recorded body temperatures in four runners, two squash players and one swimmer at 1-min intervals using miniature data loggers. These single-channel loggers are small and light (25 g), and were easily carried by the athletes during their sporting activities. Wide-range loggers (-37 degrees C to +46 degrees C), which had a temperature resolution of 0.4 degrees C, were used to measure thigh skin temperature. Auditory canal temperature and rectal temperature were measured with narrow-range loggers (+34 degrees C to + 46 degrees C) which had a considerably higher resolution (0.04 degrees C). With the aid of visual analogue scales subjects reported that the thermometric equipment caused very little discomfort or impairment of exercise performance. Loggers connected to uncoated bead thermistors (used for skin and auditory canal temperatures) had a thermal time constant of 0.4 s, and that of the coated thermistors (rectal probes) was 6 s. We were able to waterproof the equipment and measure rectal temperature in a swimmer. Hot (35 degrees C) or cold (5 degrees C) ambient temperatures had an insignificant effect on the intrinsic accuracy of the data loggers, even when used without recalibration at those temperatures. We believe that miniature temperature loggers are convenient and accurate thermometers for use during sporting activities and may provide new insights into thermoregulation during exercise.