This chapter describes in vivo methods to assess fetal amino acid and protein metabolism. The most basic techniques include chemical composition analysis of the fetus and fetal blood sampling for amino acid and protein concentrations. These methods have been used in animal models and in humans on a selective basis. In large animal models, application of the Fick principle has been used to quantify net placental and fetal uptake and production rates of amino acids. Most recently, tracer 178methodology has been used to measure placental uptake, metabolism, and transfer to the fetus of individual amino acids and their metabolic products, as well as the metabolism of these amino acids in placenta and fetus to other amino acids, protein synthesis, protein breakdown, oxidation, and glucogenesis. A recent review of nitrogen, amino acid, and protein metabolism in the placenta and fetus provides a summary of data derived from animal models and humans studied by methods described in this chapter. 1