Rate of heat storage (S) was measured by using direct and indirect calorimetry simultaneously in 11 subjects during rest and exercise at three ambient temperatures (Ta of 20, 25, and 30 degrees C), and at two work intensities (40 and 90 W). At rest, the mean value of S was -64.9 W at 20 degrees C, -26.1 W at 25 degrees C, and +9.9 W at 30 degrees C. After 50 min of exercise at 40 or 90 W, S tended toward zero at the three ambient temperatures. This indicates that thermal equilibrium was reached. In addition, at the end of the exercise periods total heat losses (R + C + E) measured at a Ta of 20, 25, and 30 degrees C were similar, i.e., independent of Ta. During the thermal transients and the steady state of exercise, the calorimetric method allows immediate measurement of S to be made, since all the physical terms of the body heat balance equation are determined. The changes in mean body temperature (delta Tb) measured by thermometry showed a delay of 5–10 min when compared with delta Tb measured by calorimetry. Thus, determination of delta Tb by thermometry is not directly applicable during thermal transients, unless the observed delay is taken into account. Our results also support the concept that Tb may be the regulated variable of the thermoregulatory system, since we obtained a very significant and uniform correlation between Esk and delta Tb at the three Ta and the two work intensities which were studied.