Twelve crossbred sows were used in an energy balance study to estimate energetic efficiency of milk production from feed. Balances were made from 7 to 14 d and from 18 to 25 d of lactation. Two feeding levels were applied (high and low). The low level (L) was meant to supply energy slightly above maintenance, with energy needed for milk to be derived from body reserves. The high level (H) was meant to supply sufficient energy for maintenance and for milk production. The low-level animals received 2.5 to 2.6 kg of feed/d and the high level animals 4.8 to 6 kg/d. The loss of weight of sows during lactation depended more on feeding level than on stage of lactation. A high level of feeding to sows resulted in heavier piglets compared with the low feeding level (significant after 10 d). At 24 d of age piglets with the high-feeding-level sows weighed 7.5 kg and those with the low-level sows, 5.7 kg. At the high level, animals excreted 1,200 to 2,800 kcal more milk energy per day than the low-level animals. Energy for milk from feed was produced with an efficiency of 67 to 69% with a maintenance requirement of 112 to 125 kcal metabolizable energy (ME) X W-.75 X d-1. Efficiency of milk production from feed was calculated as 62% and the maintenance requirement was 68 kcal. In this calculation, milk was corrected toward zero energy balance. Another way of calculating this efficiency after correcting feed toward zero energy balance resulted in estimates of 68% for efficiency and of 88 kcal ME X W-.75 X d-1 for maintenance requirement. From these data it was derived that, for each piglet, the sow needed to receive .5 to .6 kg of extra feed (ME content 3,000 kcal/kg) per day to cover milk production. Level of metabolic rate for nursing piglets was estimated as 97 kcal ME X W-.75 X d-1 for maintenance and, in addition, .195 kcal/kcal extra of milk intake above maintenance.