The objective of this study was to determine the threonine requirement for maintenance in adult sows using the indicator oxidation technique. Four non-pregnant Hypor Hybrid (Hypor Inc) sows (233.0±18.6 kg BW) were adapted to 2.2 kg of a semi-synthetic diet containing 14.0 MJ ME/kg and 3.77 g/kg threonine. Each sow received 6 test diets, in random order, providing threonine intakes of 20, 40, 60, 100, 120 and 140 mg/kg^0.75. 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 threonine intake increased up 100 mg/kg0.75 BW, and did not change at greater threonine intakes. Maintenance threonine requirement, calculated by breakpoint regression analysis, was 98 mg/kg^0.75. Heat production was linearly correlated (r = 0.70, P = 0.001) with PHE oxidation. Heat production responded in a quadratic manner to threonine intake, and was minimized at a threonine intake of 72 mg/kg^0.75. The mean respiratory quotient (RQ, 1.067±0.011) showed that dietary energy was not limiting. RQ was greatest (P<0.001) at threonine intake of 87 mg/kg^0.75.  The reduced heat production and increased RQ at threonine intakes close to the requirement indicate that maximum energetic efficiency is achieved when both amino acid deficiency and excess are avoided.