The objective of this study was to determine the lysine requirement of a population of 2nd and 3rd parity sows in early- (day 24 – 45)
and late- (day 86 – 110) gestation. Pregnant Hypor Hybrid (Hypor Inc) sows (n=7; 185.7±9.6 kg BW) were  adapted to individual intakes of a semi-synthetic diet containing 14.0 MJME/kg. Each sow received 6 different test diets, in random order, from 60 –150% of the requirement suggested by NRC (1998) in early- and from 60 –185% of the requirement suggested by NRC (1998) in late-gestation. Actual dietary lysine intakes ranged from 7.5 – 19.3 g/d in early- and 8.1 – 23.7 g/d in  late-gestation. Differences between the dietary lysine requirement for earlyand late-gestation was determined by measuring oxidation of the indicator amino acid L-[1-13C]-phenylalanine (Phe) during a primed-constant oral infusion. The average number of piglets born alive was 13.7±1.9, but ranged from 4 to 20. The average piglet birth weight was 1.5±0.1 kg. Sows gained 600 g/d from breeding and weighed 258.8±8.3 kg before farrowing. Breakpoint analysis of Phe oxidation indicated that the lysine requirement of 2nd parity sows was 13.1 g/d and 18.7 g/d in early- and late-gestation, respectively. For 3rd parity sows, the dietary lysine requirement was 8.2 g/d and 13.0 g/d for early- and late-gestation, respectively. Phase feeding at least two diets would improve productivity by more correctly providing the necessary nutrients for the growth of the placenta, the piglets, and the mammary gland.