Production of ammonia N, a-amino N, and peptide N was investigated following in vitro ruminal incubation of solvent soybean meal (SBM), dehydrated alfalfa, corn gluten feed, fish meal, distillers dried grains with solubles (DDG), cotton seed meal, brewers fried grains, meat and bone meal, blood meal, prolac, and casein (CAS). The influence of milling procedures on the production of ammonia N, a-amino N, and peptide N was also evaluated using different batches of soybean meals and distillers dried grains with solubles. The concentrations of peptide N and ammonia N measured in the cell free media at 0, 2, 4, 6, and 8 h were increased linearly (P < 0.001) with time. The mean concentrations of a-amino N were lower (P < 0.05) than the mean concentrations of peptide N and ammonia N. Production of peptide N, a-amino N, and ammonia N were varied (P < 0.05, time x protein) between proteins and between batches. Irrespective of the protein used, the amino acid composition of peptides (<3,000 MW) that appeared at 8 h had specific patterns suggesting differential utilization of peptides by ruminal microorganisms. Cell-free supernatants obtained following incubation (8 h) of SBM, CAS, and DDG were used as mucosal substrates in parabiotic chambers to quantify absorption of free and peptide-bound amino acids via ruminal and omasal epithelia of sheep. Serosal appearance of amino acids in peptide form was nearly three times higher (P < 0.001) than free amino acids. On tissue dry weight basis, serosal appearance of amino acids was greater (P < 0.01) across omasal than via ruminal tissues. There was a greater serosal appearance of amino acids from CAS than from SBM. Total, total essential (EAA), total nonessential (NEAA), and individual amino acid appearance in serosal fluids varied (P < 0.05, amino acid form x protein source) among SBM, CAS, and DDG. Collectively, these results indicate that the forestomach epithelia of sheep possess the potential to absorb ruminally derived peptides (relatively large amounts) and free amino acids (relatively small amounts). Also, the ruminal microbial degradation of dietary proteins may influence the amounts and types of free and peptide-bound amino acids absorbed via forestomach.