A mathematical model of the citric acid cycle devoted to the analysis of 13C-NMR data was developed for determining the relative flux of molecules through the anaplerotic versus oxidative pathways and the relative pyruvate carboxylase versus pyruvate dehydrogenase activities. Different variants of the model were considered depending on the reversibility of the conversion of fumarate into malate and oxaloacetate. The model also included the possibility of orientation-conserved transfer of the four-carbon citric acid cycle intermediates, leading to conversion of succinyl-CoA C1 into either malate C1 or C4. It was used to analyse NMR data from glutamine isotopomers produced by cerebellar astrocytes incubated with [1-13C]glucose. Partial cycling (39%) between oxaloacetate and fumarate was evident from the analysis. Application of the model to glutamate isotopomers from granule cells incubated with [1-13C]glucose [Martin, M.. Portais, J.C.. Labouesse. J., Canioni. P, & Merle, M. (1993) Eur. J. Biochem. 217, 617-625] indicated that total cycling of oxaloacetate into fumarate was, in this case, required to get the best fit. The results emphasized some important differences in carbon metabolism between cerebellar astrocytes and granule cells concerning the sources of carbon fuelling the citric acid cycle and the carbon fluxes on different pathways.