The objective of this study was to determine the Trp requirement in EG (d 35 to 53) and LG (92 to 111) using the indicator amino acid oxidation method. The same 6 second parity sows were studied in EG and in LG at a constant ADFI of 2.4 kg/d. Each sow received 6 diets based on corn, corn starch and sugar in both EG and LG at constant ADFI of 2.5 kg/d. Diets in EG contained Trp at 20, 40, 60, 80, 100 and 120 % of the Trp requirement (2.5 g/d) in EG, for sows of similar BW, maternal gain and litter size, and 60, 80, 100, 140, 160 and 180% in LG. The 13C background in expired CO2 and plasma free Phe was determined for 1.5 h. Sows were fed 2 mg/(kg BW·h) of L[1-13C]Phe over 4 h in 8 ½-hourly meals. Expired CO2 and plasma free Phe were analyzed for 13C enrichment above background. The Trp requirement of pregnant sows was 1.7 g/d (P = 0.001) in EG and 2.6 g/d (P = 0.016) in LG. Phe retention increased (P = 0.001) from EG to LG, and Phe oxidation (P = 0.029) and body protein breakdown (P = 0.017) decreased from EG to LG. Phe retention and oxidation responded quadratically (P = 0.038) to increasing Trp intake. The changes in Trp requirement and energy retention during gestation cannot be adequately met by increasing the feed allowance of a single diet throughout pregnancy. Therefore, phase feeding of 2 diets with different Trp contents is necessary to balance Trp and energy intake with the changing Trp and energy requirements in pregnancy.