Background:Over the last decade, the nutritional requirements of lactating mod genotype sows have increased. The current nutritional recommendations might be unable to meet the needs of increased litter size and milk production, and thus the nutritional requirements need to be re-evaluated. The current study was conducted to investigate the effects of dietary methionine to lysine (Met:Lys) ratios on theperformance of and methionine metabolism in lactating sows. Results: During the 1st week of lactation, piglets reared on sows in the 0.37 to 0.57 Met:Lys ratio groups grew faster than those reared on sows in the control group (0.27) (P<0.01). The 0.37-ratio group showed increased levels of GSH-Px in plasma during lactation (P<0.01) and decreased concentrations of urea nitrogen in the plasma of sows (P<0.05). Compared with the 0.27-ratio group, the levels of T-AOC and GSH-Px in the plasma and homocysteine in the milk of lactating sows were significantly increased in sows in the 0.47-ratio group (P<0.01). In sows fed a 0.57-ratio diet, the levels of glutathione and taurine in the plasma and milk were improved significantly during lactation. However, the content of TBARS in the blood (P<0.05 at day 7 and P=0.06 at weaning day) was increased (P<0.01). Moreover, there were linear increases in the levels of homocysteine in the blood and milk of sows during the lactation period (P<0.01) with increased dietary Met:Lys ratios in the lactation diet. Conclusions:The current study indicated that increasing the dietary Met:Lys ratio (0.37～0.57) in the lactation diet had no significant effect on the overall performance of sows or the colostrum and milk composition, but it increased piglet mean BW and piglet ADG during the first week of lactation. Increasing dietary methionine levels had no significant effect on antioxidant function in lactation sows, even though it increased levels of GSH and GSH-Px in the plasma of sows during lactation. However, the content of homocysteine in the plasma and milk increased during lactation due to a high level of dietary methionine.