This study was conducted to determine the Trp requirement in EG and LG using the indicator AA oxidation method. The same 6 2nd parity sows were studied in EG (d 35 to 53) and in LG (d 92 to 111) at a constant feed amount of 2.4 kg/d. Each sow received 6 diets based on corn, corn starch and sugar in both EG and LG. 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 body weight, maternal gain and litter size, and 60, 80, 100, 140, 160 and 180% in LG. Sows were fed 2 mg/(kg BW·h) of L[1-13C]Phenylalanine (Phe) over 4 h in 8 ½-hourly meals. Expired CO2 and plasma free Phe were analyzed for 13C enrichment above background. Requirements were determined as the breakpoint in 2-phase nonlinear models. Sows grew from 167.7 kg (SE 3.93) at breeding to 211.9 kg (SE 5.18) post farrowing. The sows had litters of 14.5 piglets (SE 0.43) weighing 19.0 kg (SE 1.41) at birth. The Trp requirement was greater (P = 0.002) by 52% in LG (2.6 g/d) compared to EG (1.7 g/d). 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 increase (P = 0.001) 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. 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.