Pregnant adult sows show little maternal growth so that accelerated conceptus growth in the 3rd trimester can cause large differences in nutrient requirements between early (EG) and late (LG) gestation. The objective of this study was to determine the Ile requirement in EG (d 37 to 61) and LG (d 89 to 109) using the indicator amino acid oxidation (IAAO) method.  It was found that Ile requirements were driven mainly by maintenance in EG and by fetal growth in LG.  Both Ile and energy intake must be increased in LG but the greater change in Ile than energy requirement shows that amino acid supply is more critical in LG for adult sows.