Accelerated conceptus growth in the last trimester may increase amino acid and energy requirements of pregnant sows. The objective of this study was to determine the Tryptophan (Trp) requirement in early (EG, d 35 to 53) and late (LG, d 92 to 111) gestation using the indicator amino acid oxidation (IAAO) method. It was found that the The Trp requirement was greater by 52% in LG (2.6 g/d) compared to EG (1.7 g/d). The increase in Phe retention from EG (2.94 g/d) to LG (8.28 g/d) agreed with a gain of 1 g/d N per fetus in LG and indicated that maternal protein gain was similar in EG and LG. Heat production was greater by 3% in LG compared to EG. Lipid retention decreased from EG to below zero in LG. Young sows strive to maintain fetal and maternal protein growth even if lipid retention becomes negative. This shows the importance of meeting amino acid requirements in late pregnancy.