Nitric oxide (NO) is an important gaseous radical involved in many physiological processes. It is produced from the amino acid l-arginine by the action of nitric oxide synthases (NOS) in what is called the l-arginine/NO pathway. Tracking its metabolic fate in biological fluids is of particular interest as it may indicate how the human body responds in health and disease. However, due to its short life span (a few seconds) it is very difficult to accurately monitor any up- or down-regulation in body fluids in vivo. As a consequence, methods have been developed based on the measurement of the NO-derived products nitrite and nitrate or on the substrate of NO, l-arginine and its simultaneously generated product, l-citrulline. Considering only a fraction of the endogenous l-arginine pool is used for the synthesis of NO, NO-production cannot be estimated by measuring changes in the concentrations of l-arginine and/or l-citrulline alone. Instead, to estimate NO-related changes in the l-arginine and/or l-citrulline pools a form of tagging these metabolites for the NOS-mediated reaction is required. The application of stable isotopes is an elegant way to track NOS-mediated changes. The present paper is focussed on the application of various combinations of chromatography and mass spectrometry to measure isotopic enrichments resulting from the conversion of l-arginine to NO and l-citrulline in a one-to-one stoichiometry. In addition, the various aspects and principles involved in the application of stable isotopes in metabolic studies in general and the study of the activity of NOS in particular are discussed. AD - Department of Surgery, University Maastricht, P.O. Box 616, NL-6200 MD Maastricht, The Netherlands.