A total of 335 lactating sows (Landrace × Large White) were used in two experiments to determine the optimum ratio of standardized ileal digestible lysine (SID-Lys) to metabolizable energy (ME) for mixed parity sows during lactation.In Exp.1,185 sows (weighing an average of 256.2 ± 6.5 kg and having an average parity of 3.4 ± 0.3)were allocated to one of six experimental diets in a completely randomized block design within parity groups (1,2,and 3+).The experimental diets were formulated to contain 3.06,3.16,3.20,3.25,3.30 or 3.40 Mcal/kg of ME and each diet was fed to the sows throughout a 28 day lactation.All diets provided a similar SID-lysine level (0.86％).As a result,the diets provided a SID-Lys:ME ratio of 2.81,2.72,2.69,2.65,2.61 or 2.53 g/Mcal ME.Sow feed intake was significantly (P ＜ 0.01) affected by the energy content of the diet as well as by sow parity.Using regression analysis,feed intake was shown to be maximized at 3.25,3.21,3.21 and 3.21 Mcal/kg of ME for parity 1,2,3+ sows and the entire cohort of sows respectively (quadratic; P ＜ 0.01).In addition,the result of feed intake can be expressed as 2.65,2.69,2.69 and 2.68 g/Mcal based on analysis of SID-Lys:ME ratio.Litter weight gain was affected by dietary treatment for parity 3+ sows and the entire cohort (P ＜ 0.01).Based on regression analysis,litter weight gain was maximized at 3.25 and 3.24 Mcal/kg of ME for parity 3+ (quadratic; P ＜ 0.01) and the entire cohort (quadratic; P ＜ 0.01).Similarly,the result of litter weight gain could be expressed as 2.65 and 2.66 g/Mcat of SID-Lys:ME ratio.Therefore,3.25 Mcal/kg of ME was selected for Exp.2 in which 150 sows (weighing 254.6 + 7.3 kg and having an average parity of 3.4 ± 0.4) were allocated to one of five treatments in a completely randomized block design within parity (1,2,and 3+).The experimental diets were formulated to contain 2.1,2.4,2.7,3.0 or 3.3g/Mcal of SID-Lys:ME ratio with all diets providing 3.25 Mcal/kg of ME.The diets were fed to the sows throughout a 28 day lactation.Sow body weight loss was affected by dietary treatment (parity 3+ sows,P =0.02; entire cohort,P ＜ 0.01) and by sow parity (P ＜ 0.01).Litter weight at weaning and litter weight gain were affected by dietary treatment for parity 1,2,3+ sows and the entire cohort (P ＜ 0.01) as well as by sow parity (P ＜ 0.01).Plasma urea nitrogen (P ＜ 0.01),creatinine (P ＜ 0.01) and non-esterifide fatty acids (P =0.04) were decreased as the SID-Lys:ME ratio of the diet increased.Insulin-like growth factor-1 (P =0.02),estradiol (P ＜ 0.01) and luteinizing hormone (P =0.02) were increased as the SID-Lys:ME ratio in diet increased.Based on a broken-line model,the estimated SID-Lys:ME ratio to maximize litter weight gain was estimated to be 3.05 g/Mcal.