A ruminal fermentation study was used to investigate the relationship between forage quality and monensin-dependent amino acid-sparing. Two fistulated cows were fed three concentrations of chopped (2.5 to 5.0 cm in length) timothy and alfalfa hays (100:0, 50:50 and 0:100) and two levels of monensin (0 and 350 mg.cow-1.d-1). The diets were offered 12 times per day (9 kg of DM/d), and the rumen reached a steady-state, reducing animal and day variation. Alfalfa hay had 1.4 times more CP and 1.4 times less NDF than timothy hay, and the substitution of timothy with alfalfa increased (P < .05) ruminal ammonia. Monensin had no effect on total ruminal ammonia when timothy hay was present in the diet, and it increased total ruminal ammonia with the 100% alfalfa diet. Effects on ruminal ammonia were, however, confounded by monensin-dependent decreases in ruminal pH (P < .05). Dissociated ammonia, the species most likely to be adsorbed from the rumen, declined when monensin was added to the 100% timothy diet (P < .05). Monensin had no effect on dissociated ammonia if alfalfa was 50% or 100% of the forage, but it counteracted alfalfa-dependent decreases in bacterial protein (P < .05). The idea that monensin could spare amino acids was supported by the observation that monensin decreased (P < .001) the specific activity of deanimation and increased bacterial protein at all combinations of alfalfa and timothy. Increases in bacterial protein could be explained by monensin-dependent increases in total VFA.