Energy metabolism and leucine oxidation were measured in lactating sows as part of a long-term research program designed to redefine the protein, amino acid, and energy requirements of modern, high producing sows. Leucine  oxidation was used to estimate the catabolism of dietary and body protein for energy. The data showed that sows actively stored energy through lipogenesis during the overnight period, but were forced to utilize those reserves during the day when piglets were most actively suckling. Leucine oxidation was greater (P<0.0001) on d 17 than d 7 (28.8% vs. 21.8% of dose). Milk (5.4 kJ/g; 5.2% crude protein) production per piglet (656 g/d vs. 940 g/d) increased (p<0.05) while feed intake (kg/d) tended (P<0.10) to increase from early to late lactation. In conclusion, feed intake during lactation was insufficient to meet energy requirements of these sows, based on measurements of energy metabolism. Amino acid oxidation increased between early and late lactation, indicating that more protein was being catabolized, probably to provide additional energy. Therefore, the amino acid intake of these sows in late lactation was insufficient to meet the demands for both protein synthesis and energy utilization.