The objective of this study was to determine the Ile requirement in early (EG, 37 to 61 d) and late (LG, 89 to 109 d) gestation using the indicator amino acid oxidation method. Seven 4th parity sows were used in EG and LG. 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 Ile at 20, 40, 60, 80, 100 and 120 % of the Ile requirement (6.2 g/d, NRC 1998) for sows of similar BW, expected 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. Requirements were determined as the breakpoint in 2-phase nonlinear models. Sow BW was 246.5 kg in EG and 271.6 kg in LG. ADG was similar in EG (344 g/d) and LG (543 g/d). Sow maternal gain was 19.1 ±4.4 kg and litters of 17.7 ± 0.75 piglets weighed 22.6 ± 0.87 kg at birth. Energy retention was similar in EG and LG, but the respiratory quotient decreased (P = 0.047) from EG (1.05) to LG (0.98) and decreased (P = 0.016) with increasing dietary Ile level, indicating lipid mobilization in LG when Ile was at or above the requirement. The increase in Ile requirement from EG to LG suggests that phase feeding during gestation is necessary. Diets for LG should contain more Ile and be fed at greater allowances than in EG to meet the sows’ demands for nutrients.