Milled (13 mm screen) maize residue containing 35% moisture was treated with 4% anhydrous ammonia on roughage dry matter in a stack for 50 days. Performance of lambs was measured in a 2 x 5 factorial experiment with two treatments (untreated and ammoniated) at five concentrate levels (0, 20, 40, 60 and 80%). Digestibility and rumen characteristics were measured in 10 ruminally cannulated wethers in two 5 x 5 Latin square designs. Rumen volume and flow rate were determined using Cr-EDTA. Feed intake, average daily gain, and feed conversion were all improved by ammoniation at the 0 and 20% concentrate levels (P<0,05) with the differences at the 40 and 60% levels being not significant. Ammoniation led to a reduced rumen digesta mass and increased dressing percentage. The increase in in vivo digestibility owing to ammoniation was higher at low concentrate levels and lower than that measured in vitro. Rumen pH was decreased by feeding concentrate as well as by feeding ammoniated roughage at all concentrate levels (P<0,05). High rumen ammonia-N levels were recorded on ammoniated diets but N digestibility on these diets was reduced. Ammoniation tended to decrease retention time of water, rumen fluid volume, and flow rate out of the rumen.