Entry of metabolic 14CO2 into urea is shown to occur more readily than it equilibrates with the general pool of cellular plus extracellular bicarbonate plus CO2. Since the sites of CO2 production (pyruvate dehydrogenase and oxoglutarate dehydrogenase) and of fixation (carbamoylphosphate synthetase) are intramitochondrial, it is likely that the fixation of CO2 is also more rapid than its equilibration with the cytoplasmic pool of bicarbonate plus CO2. This observation may point to a more general problem concerning the interpretation of isotope data, when compartmentation or proximity of sites of production and utilisation of metabolites may result in the isotope following a preferred pathway.