A simulation model was built to represent the kinetics of endogenous-protein nitrogen (EPN) in the gastrointestinal tract (GIT) of ruminants. The GIT is split into three compartments, namely: stomach, small intestine and large intestine. The model contains three pools corresponding to these compartments and utilizes two types of source (secretions and inflow) and exit (absorption and passage) for EPN. Parameter values are set based on relevant data from the literature. According to model predictions, proportion of EPN in the total N flow increases as CP content of the diet and nitrogen intake decrease. On extremely low protein diets, EPN may represent 43, 66 and 92% of the total N in the abomasal, ileal and faecal contents, respectively. The model can contribute to improve knowledge on nitrogen recycling as well as to account for endogenous protein losses in the calculation of maintenance requirements.