A radiorespirometer was constructed for continuous quantitation of "CO, released from specifically labeled substrates by intact cultured cells attached to plastic petri dishes. An airtight chamber is created by sealing the petri dish with a specially designed cover inside a thermostated holder. Rapid equilibration of released 14COI with a 5% COJ95% air carrier gas is achieved by bubbling the carrier gas under the surface of the growth medium. Labeled CO* is removed from the carrier gas by trapping in an organic base and quantitated by liquid scintillation counting. Additions to or sampling of the growth medium may be performed during a run and the carrier gas may be modiied to test the effects of anesthetics and different 0% levels. The ability to continuously monitor YO, release can provide valuable information concerning the metabolic pathways of substrate oxidation which cannot be obtained from single *'COs determinations. A capacity of 12 culture plates enormously increases the amount of data that can be collected in a given time. The use of liquid scintillation counting increases the sensitivity and resolution over the ion chamber and Geiger counter methods, and permits utilization of the procedure in a much wider range of laboratories. Data obtained for the oxidation of specifically "C-labeled glucose and pyruvate by neonatal rat heart cells in culture, in both the presence and absence of oxygen, are provided as examples of the utility of the method.