Measurements of heat production(HP; indirect calorimetry) and its partition between maintenance, physical activity, thermoregulation, and thermic effect of feed or energy gain were carried out in sows maintained in different situations: primiparous or multiparous; pregnant or nonpregnant; thermoneutral or cold conditions; varied feeding levels; and varied body weights (BW). Metabolizable energy requirements for maintenance average 420 kJ/kg BW.75 at thermoneutrality and moderate physical activity. This value is not significantly affected by parity, pregnancy, and stage of pregnancy. Physical activity is a major factor causing differences in energy balance between sows because activity is variable and its energy cost (27 kJ.kg BW-.75.100 min-1 standing) is four to five times higher than in other species. Lower critical temperature (LCT) is approximately 20 degrees C in pregnant and individually housed sows, and daily HP is increased by approximately 15 kJ/kg BW.75 for each degree Celsius decrease of ambient temperature below LCT. Efficiencies of utilization of ME for meeting energy requirements for maintenance, maternal gain, and uterine gain are 77, 75, and 50%, respectively. Equations for predicting energy deposition in the uterus and mammary gland are proposed. In addition to activity and thermoregulation, energy requirements of pregnant sows depend on body reserves of energy. Studies with newborn pigs indicate that they are quite sensitive to ambient temperature (i.e., +25 J.kg BW-.75.min-1 for each degree Celsius decrease of temperature), and their LCT is 32 to 34 degrees C. The energy demand of pigs for thermoregulation just after birth relies mainly on carbohydrates from glycogen reserves or colostrum. Survival of newborn pigs is highly dependent on the supply of colostrum.