The possibilities of estimating parameters for whole body protein metabolism (synthesis and breakdown) by the help of the end product method and a compartment model using ¹⁵N- and/or ¹⁴C labeled amino acids (AA) as tracers are demonstrated. First, the fundamentals of the method, the choice of tracer AA and the determinable parameters are described. The influence of endogenous and exogenous factors on protein synthesis and breakdown is demonstrated using selected results of tracer experiments on rats. Differences relating to animal species and the effect of age, body weight, protein intake, β-agonist administration and training, are shown in detail. The main conclusions are: - the tracer method in combination with modeling provides parameters of protein metabolism on intact animals; - the estimated data are relative values and they reflect the main processes of protein and/or AA metabolism with sufficient accuracy; - the method makes it possible to assess the influence of different endogenous and exogenous factors on the protein synthesis rate.