Protein, fat, carbohydrate and energy metabolism were studied in 8 lactating sows with their litters. Intake of nutrients and energy and excretion in faeces, urine and milk were studied in digestibility and nitrogen (N) and carbon (C) balances, while indirect calorimetry was used to quantify heat (HE) and methane (CH₄) production. As the piglets have to join the sow during measurements total HE (sow + piglets) was measured. In order to separate total HE into sow HE and piglet HE different methods were used. The sow milk production was measured by deuterated water (D₂O) dilution. By using doubly labelled water (D₂¹⁸O) in three piglets per litter, the CO₂ production of the piglets was measured simultaneously with milk production with high accuracy. The sow HE was measured from heart rate by implantation of a telemetric transmitter. In addition total HE could be separated into sow HE and piglet HE by dividing the gas exchange according to metabolic liveweight (LW, kg^0.75) of sow and piglets and by using factorial methods for sow and piglets. The physical activity of the sow was recorded by means of a photocell. Advantages and disadvantages by using the different methods are discussed.