A total of sixty-five observations on heat production during fasting and physical activity were obtained in four groups of pigs differing in breed and/or castration (Meishan (MC) and Large White (LWC) castrates and Large White (LWM) and Pietrain (PM) males) with body weight (BW) ranging between 25 and 60 kg. Pigs were fed ad libitum before fasting. Heat production was measured using indirect calorimetry. Fasting heat production (FHP) was proportional to the body weight raised to the power 0.55, but with group-specific proportionality parameters (8 10, 1200, 1220 and 1120kJ/kg BW@55 per d for MC, LWC, LWM and PM respectively). Group effects could be removed by expressing Fflp as a function of muscle, viscera and fat: FHP affected the number of bouts of physical activities (i.e. standing or sitting) per day, the duration of activity and the total cost of activity. Energetic cost of activity was proportional to the muscle mass raised to the power 0.91 (FHPaCtivity (kJ/h activity) = 21.0(mu~cle)~'~~). Physical activity represented less than 10 % of the total heat production in fasting growing pigs housed alone in metabolic cages and kept in a quiet environment.