This experiment was conducted to evaluate the effects of different sources of rumen degradable amino acids, soybean meal versus a commercial source of hydrolyzed cottonseed cake with low molecular weight (Fortid ® ), on the in vitro ruminal fermentation responses and the microbial nitrogen yield of glucogenic (n=8) and lipogenic (n=8) diets. An in vitro gas production technique was used to determine the differences in microbial nitrogen yield of the diets. Sources of grain including barely and corn were used in the glucogenic diets as grinded or steam flaked, while in the lipogenic diets, corn silage, sugar beet pulp, linseed and prill fat were used to provide the energy needed for rumen fermentation. The experiment was a randomized design including two types of diets × two sources of rumen degradable amino acids factorial arrangement. The gas produced from the fermentable fraction (b) was higher for the lipogenic diets containing corn silage and F than those for the others. The fraction (b) and microbial nitrogen to diet nitrogen ratio were greater (p<0.05) for Fortid ® versus soybean meal. The true substrate digestibility of glucogenic diets were significantly higher (p<0.05) compared to those of lipogenic diets. Both the grain sources used as steam flaked caused an increase in the rate of gas produced (c) and microbial nitrogen to diet nitrogen ratio. There were significant interactions (p<0.005) of the type of diets with either soybean meal or Fortid ® on microbial nitrogen to diet nitrogen. The results indicated that pre-hydrolyzed protein sources which are rich in low molecular weight peptides may be considered as candidates to improve rumen feed fermentation and microbial nitrogen production with both glucogenic and lipogenic diets. This conclusion was arrived at from the different patterns of action of soybean meal and Fortid ® in the present in vitro rumen fermentation responses.