In metabolic studies, the exposure of a biological system (e.g. a tissue slice, a cell culture, mitochondria) to a radiolabelled metabolite over a relatively short time, in order to subsequently follow the passage of the label from precursor metabolite to product metabolite. A variant is pulse-chase labeling, in which the pulse is terminated by the addition of a large amount of the unlabelled metabolite in order to rapidly lower its specific radioactivity.