The stable nitrogen isotope15N is being increasingly used in life sciences. Problems of following the course of nitrogen in digestion/absorption studies of nutrients, their intermediary metabolism and dynamic behaviour can be readily dealt with using nitrogen-15. The object of this paper is to discuss some aspects of the isotope analysis of15N, useful tracer techniques, and theoretical considerations. It is shown that nitrogen balance is a valuable concept for expressing the relationship between the overall nitrogen intake of the body and its nitrogen losses. However, a mere summary of total exchange gives little information of any particular metabolic processes within the body unless these are reflected in the total nitrogen exchange. Feeding trace doses of15N labelled substances like proteins and amino acids the kinetics of digestion and absorption is indicated by a measurable change of the15N abundance in serum and urinary nitrogen compounds, i.e. by a significant delay in nitrogen absorption in patients with gastrointestinal diseases.