The recommended daily lysine requirement of non-pregnant sows at maintenance, 36 mg/kg, was estimated from research in the 1960’s with growing pigs and sows. The current estimate was hypothesized to be too low for modern, high producing sows.  Non-pregnant Hypor Hybrid (Hypor Inc) sows (n=4; 222.6±0.7 kg BW) were adapted to 2.2 kg of a semi-synthetic diet containing 14.0 MJ ME/kg and 1.09 g/kg lysine. Each sow received 6 test diets, in random order, providing lysine intakes of 19.8, 25.2, 30.6, 41.4, 46.8 and 52.2 mg/kg. After adaptation to each diet, indicator amino acid oxidation using L-[1-13C]-PHE was determined simultaneously with indirect calorimetry for 4 h. Plateaus in oxidation were achieved within 1.5 h. PHE oxidation decreased as lysine intake increased and was lowest (P<0.05) at 46.8 mg/kg. Heat production was linearly correlated (r2 = 0.54) with PHE oxidation and was lowest (P<0.05) at lysine intake of 46.8 mg/kg. The mean RQ (1.029±0.028) showed that dietary energy was not limiting. RQ was also lowest (P<0.001) at lysine intake of 46.8 mg/kg. Maintenance lysine requirement, calculated by regression analysis, was 49 mg/kg.  The dietary lysine requirement for this population of sows was determined to be 49 mg/kg; this exceeds the current recommendation by 30%. HP and RQ were lowest when sows received adequate intake of dietary lysine, demonstrating that energy metabolism was also most efficient at this intake. Amino acid requirements of modern sows must be redetermined during maintenance, gestation and lactation.