The effect of mixing on energy metabolism was studied in 8-wk-old pigs. In each of two trials, two clusters of 20 pigs (two litters of 10 pigs) were randomly assigned to one of two treatments: control or mixing. Each cluster was housed in two pens. In each trial, after a preliminary period od 2 wk, the two litters within the mixing treatment were mixed at the start of a 2-wk experimental period. During mixing, the five heaviest pigs of each litter were put together in one pen, and the five lightest pigs of each litter were put together in the other pen. In the control treatment, the social structure of both litters in one climatic chamber was not altered. After mixing, a short-term effect on total heat production and activity-related heat production was present. Both were increased (P < .01) only during the 1st h after mixing. Only 57.3% of this increased total heat production was caused by an increased activity. However, no long-term effects of mixing on energy partitioning were present during the total experimental period. The absence of a long-term mixing effect might be caused by the optimal conditions at the moment of mixing. In the preliminary period the transposition of GE into ME increased 1.3% (P < .05), and ME for maintenance decreased 80 kJ.kg(-.75).d(-1) (P < .01) between wk 1 and 2. These large alterations in energy metabolism are probably a carry-over effect of the transportation of the pigs and (or) the changes in housing environment.