Nuclear magnetic resonance (NMR) spectroscopy is a non-invasive means of detecting numerous compounds, thus enabling real-time studies to be carried out on such topics as energy metabolism, intracellular pH and main intermediate metabolism pathways. This method has been used for several years to study perfused livers and rat livers in vivo, but its application to man has just begun. We describe some of its principal uses, with emphasis on the possibilities it offers to investigate the metabolic pathways. With the rapid advances made in this field, and particularly spectroscopic imaging, NMR spectroscopy should prove valuable in four main branches of research: understanding of physiological and pathological metabolic mechanisms, diagnosis with the possibility of functional tests, follow-up and assessment of new medical treatments, and pharmacology.