Recent data suggests that gestating sows have a greater capacity to digest dietary protein than growing pigs and thus different values for amino acid availability should be used when formulating sow diets. The MA of threonine in corn and barley in six gestating gilts (146 ± 4 kg BW) was determined using the indicator amino acid oxidation technique. A paired study using six growing pigs (146 ± 4 kg BW) and corn was conducted to allow a direct comparison between a) determined MA values and published TID values for growing pigs and b) the determined MA for sows and growing pigs. Pigs were fed reference diets with increasing synthetic THR from 50 – 80 % of requirement; all other nutrients were provided in excess. Test ingredient diets were formulated to 80 % of requirement by inclusion of corn or barley. On collection day, L [1-13C] and L [1-14C] phenylalanine (sows and growers, respectively) was given orally in eight ½-hourly meals and expired 13CO2 and 13CO2 was quantified. The Mixed model in SAS was used to determine the change (slope) in indicator oxidation per gram of total dietary THR from reference and test diets. The MA of THR was calculated as the slope for THR from test diets divided by the slope for THR from reference diets.  The MA method gives results not different from TID. The bioavailability of amino acids in feedstuffs is greater in sows than in growing pigs.