To determine the changes in thyroid hormone metabolism during short periods of exposure to heat, 30 euthyroid healthy male volunteers (aged 23--40 yr) were placed in a climatic chamber for 2 h (35 C, 50% relative humidity). The subjects were at complete rest during the first hour and performed light work (40 watts) during the second hour. Blood samples for T4, T3 and rT3 were drawn at 0, 60, and 120 min. Rectal temperature and heart rate were monitored continuously. No significant changes in T4, T3, rT3, rectal temperature, or heart rate were observed after the first hour (basal levels, 8.5 +/- 0.3 microgram/dl, 160 +/- 5 ng/dl, 14.5 +/- 2.5 ng/dl, 37.2 +/- 0.1 C, and 78 +/- 8 beats/min, respectively; mean +/- SEM). During the second hour, a significant rise in body temperature was recorded (38.5 +/- 0.1 C), accompanied by a significant decrease in mean serum T3 concentration and a rise in mean serum rT3 concentration, T4 concentration remained unchanged. Our findings suggest that, parallel to the elevation in body temperature, there is a shift in the conversion of T4 to the noncalorigenic rT3 metabolite rather than to T3.