An experiment was conducted with four preruminant calves to measure the energy cost and the diurnal pattern of physical activity in tethered, fed calves and to determine whether differences in activity could interfere with the interpretation of circadian changes in heat production. Measurements were carried out in large respiration chambers (3,650 L of inner volume), and a computation method was presented that allowed the calculation of the energy cost of standing for each standing period. This cost averaged 449 cal.kg BW-1.h(-1) (SE = 41.6, n = 4). It represented a 23 to 27% increase in heat production above that measured in the lying state. This estimate and its standard error were lower than values obtained by regression (2,131 cal.kg BW-1.h(-1), SE = 862.2, n = 8). The energy cost of standing was highest after meal times and lowest at night. These variations could reflect the nonuniform activity patterns of calves while standing. The time spent standing per hour showed the same variations during the day as the energy cost of standing. Noteworthy, the elevated energy expenditure measured in the Ist h after the morning meal was due to activity cost rather than to meal thermogenesis. Standardization of diurnal heat production profiles to a given activity pattern thus seemed to be necessary.