Treatment of oat grain by anhydrous ammonia at levels of approximately 0,75; 1,5 and 3,0% was investigated. Diets consisting of untreated oat grain and oat grain ammoniated for 24 hours at 90°C in an AN-STRA-VERTER® -oven at the respective treatment levels were individually fed at an intake level of 45 g/kg W0.75 to adult SA Mutton Merino wethers in an in vivo digestibility and N balance trial. The crude protein (CP) content of the experimental diets was 10,3; 16,0; 16,8 and 17,3% respectively. The apparent digestibility of dry matter and organic matter was significantly (P ≤ 0,01) increased from 68,1 to 69,6; 72,2 and 76,5% and from 69,1 to 71,4; 73,8 and 78,0% respectively. CP digestibility of untreated oat grain was significantly (P ≤ 0,01) improved from 72,6 to 78,4% at the 0,75% treatment level. No significant further improvement was obtained at higher ammoniation levels. Ammoniation at a level of 3,0% significantly (P ≤ 0,01) improved the apparent digestibility of neutral detergent fibre (NDF) and hemicelluloses (HC) by 22 and 29,4 percentage units respectively. The slight improvement in the digestibility of acid detergent fibre (ADF) with increased ammoniation levels was not significant. Like CP digestibility, ammoniation significantly (P ≤ 0,05) affected N balance. Ammoniation levels above 0,75%, however, resulted in no further improvement in the N balance.