The effect of temperature (23 or 33 degrees C) and feeding level on components of energy balance was studied in seven groups of individually reared Piétrain x (Landrace x Large White) littermate piglets. Within each litter, one pig was reared at 23 degrees C and given a predefined feeding level close to ad libitum (23AL pigs), one was reared at 33 degrees C and also fed close to ad libitum (33AL), and one was reared at 23 degrees C and pair-fed to the 33AL pig (23PF). Piglets of one litter were acclimated during 2 to 4 wk to their experimental temperature in temperature-controlled rooms before being transferred (one per week) to a respiration chamber for measurement of nitrogen and energy balances. The average initial BW was 22.4 kg. The data on O2 consumption, CO2 production, and physical activity were collected over seven consecutive days and used to calculate total heat production (HPtot) and its components: fasting heat production (FHP), heat production due to physical activity (HPact), and thermic effect of feed (TEF). A preliminary trial was conducted in which heat production was measured in three piglets according to a Latin square design at 23, 25, and 27 degrees C. Total heat production was, but activity-free heat production was not, affected by temperature, and no firm conclusions could be drawn as to whether 23 degrees C was within the thermoneutral zone of fed piglets. In Trial 2, the combination of increased temperature and reduced feed intake resulted in a 20% lesser heat production in 33AL than in 23AL pigs. This was due to a reduction in both TEF (-39%) and FHP (0.642 vs 0.808 MJ x d(-1) x kg BW-0.60). Despite the shorter duration of standing activity, HPact was slightly higher at 33 degrees C, probably due to hyperventilation at this temperature. With similar feeding levels (23PF vs 33AL), HPtot and activity-free heat production were less at 33 degrees C and energy retention as protein (+6%) and fat (+31%) was increased. Because HPact was similar for both treatments, the greater energy retention for 33AL seemed to be due to a greater utilization of feed energy or to a reduced maintenance requirement (i.e., reduced FHP). However, the type of stress imposed on 23PF and 33AL pigs was different and may have affected energy metabolism. The results suggest that the reduction in heat production of piglets at high ambient temperatures is caused by a reduction in voluntary feed intake and differences in energetic efficiency. The mechanisms for the lesser efficiency at 23 degrees C compared to 33 degrees C (at the same level of feed intake) remain unclear.