The energy expenditure of eight male subjects over 24 h on a fixed activity programme was measured twice at an interval of one week in a 24 m3 direct calorimeter. The standard deviation of a single measurement of the total heat output was 234 kJ, or 2.2 per cent, expressed as a coefficient of variation. The corresponding numbers for sensible and evaporative heat output were 215 kJ (3.2 per cent) and 354 kJ (9.3 per cent), respectively. The within-subjects coefficient of correlation of sensible and evaporative heat loss was -0.77. The results are in agreement with previously published results (Dallosso, Murgatroyd & James, 1982) using indirect calorimetry and show that the 24-h energy expenditure on a fixed programme is a remarkably constant quantity. Furthermore, our data show that the two components of heat dissipation, ie, sensible and evaporative heat, are negatively coupled, indicating that the body can sense and communicate the activity of both effector organs.