Rates of in situ N loss of soybean meal (SBM) were altered by various combinations of sodium hydroxide and heat treatment; some nitrogenous compounds involving the modified proteolysis were investigated. The amount of lysinoalanine (LAL), neutral detergent insoluble nitrogen, 1-fluoro-2,4-dinitrobenzene reactive lysine, and reducing power of SBM were linearly related (P < 0.01) to rate of in situ N loss with correlation coefficients of -0.847, -0.835, 0.793, and -0.846, respectively. The results indicate that the LAL formation and the Maillard reaction are related to change in susceptibility of SBM protein to microbial degradation in the rumen.