Isolated cell systems are now being used very effectively to study a range of important biochemical questions, but their energy metabolism has never been comprehensively investigated. We have developed a system, using J2E cells, which enables us to measure total ATP turnover and the contribution of various fuels and pathways to this total in a dynamic, proliferating preparation. Cells are cultured in 500 ml airtight glass containers which enables (1) the measurement of oxygen consumption, (2) the collection and measurement of 14CO2 production from labelled fuels, and (3) the measurement of metabolite utilization and production. Data on cell numbers are then used to produce a curve of cell number vs. time, the area under which (cell numbers x hour) is used as a base by which all measurements and experiments are compared. To our knowledge this is the first time a comprehensive energy budget has been measured in a proliferating cell system over a period that covers multiple cell cycles.