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 early and 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.  The dietary requirement for lysine in gestation was greater than previously reported by NRC (1998). 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.