The energy expenditure of a 2 h programme of light activity was observed daily in 8 young men after a change in habitual environmental indoor temperature of 20-24 degrees C to 34 degrees C and an absolute humidity of 6-10 g/m3 during a 10 day period. No changes were observed with respect to the daily energy intake, body weight, rectal temperature and average skin temperature, but the energy expenditure showed a decrease of between 3 and 8 per cent. The observations support results of previous studies on acclimatization with respect to energy expenditure, and indicate that differences in energy expenditure due to habitual differences in environmental temperature are small, presumably less than 5 per cent.