The objective of this study was to determine the Isoleucine (Ile) requirement for maintenance using the indicator AA oxidation method. Each of 6 sows, non-pregnant after their 4th litter, received 6 diets based on corn, corn starch and sugar at a mean intake of 2.19 kg/d (SE 0.014). Diets were calculated to provide Ile at 10, 20, 30, 50, 60, and 70 mg/kg0.75 body weight (BW). Leucine intake was 65 mg/kg0.75 BW, valine intake 62 mg/kg0.75 BW. Sows were confined in respiration chambers overnight before each oxidation study to determine fasting energy expenditure. The 13C background in expired CO2 was determined for 1.5 h. Sows were fed 2 mg/(kg BW·h) of L[1-13C]Phenylalanine (Phe) over 4 h in 8 ½-hourly meals. Requirements were determined as the breakpoint in indicator AA oxidation in 2-phase nonlinear models. Mean sow BW was 219 kg (SE 2.41). The daily gain of -0.199 kg/d (SE 0.098) during the 18-d study was not different from zero (P = 0.10). The Ile requirement was 35 mg/d×kg0.75 BW (P= 0.001). Overall Phe retention was less than zero (-0.92 g/d, SE 0.125, P = 0.001) but not different from zero (P = 0.14) for Ile intake above the requirement at -0.53 g/d (SE 0.334). Fasting heat production was 19.3 MJ/d (SE 0.41) or 334 kJ/kg0.75 BW (SE 8.5). Heat production was 31.8 MJ/d (SE 0.19) and energy retention was -0.14 MJ/d (SE 0.054), which was not different (P = 0.80) from zero. The fasting and fed respiratory quotient was 1.0 (SE 0.02) and 1.02 (SE 0.02), respectively. Because energy and protein retention were not different from zero, the value of 35 mg/d×kg0.75 represents the Ile maintenance requirement. This indicates that AA maintenance requirements are greater for modern pigs but their ratios remain similar.