The effects of feeding level and ambient temperature on time-related alterations of heat production in relation to physical activity (posture) were studied in young calves after transportation. Twenty-four 6-d-old Holstein-Friesian male calves were assigned in a 2 x 2 factorial design to one of two feeding levels (high or low) and one of two ambient temperatures (7.5 degrees C or 19 degrees C). Heat production was measured per calf continuously every 9 min by indirect calorimetry for a 2-wk period. Calf's posture during these 9-min periods was measured with a photoelectric cell. After arrival, heat production of calves decreased with time (days). The amount of decline in heat production depended on feeding level and ambient temperature. This decline with time was largest at the low feeding level at 19 degrees C. Six to 12 d after arrival, heat production reached steady-state levels. A steady-state level of heat production was reached first at the high feeding level at 19 degrees C. Averaged over treatments, heat production during standing was 132 kJ.kg-.75.d-1 higher than during lying. Time spent standing by calves increased over time. This indicated that alteration in time spent standing did not account for the time-related changes in heat production. On the contrary, physical activity (posture) partly masked the alterations in metabolic rate of calves after arrival. However, differences in physical activity (posture) accounted for differences in both time to reach steady-state levels and linear decline in heat production over time between treatments.(ABSTRACT TRUNCATED AT 250 WORDS)