A mathematical model of mammalian cell intermediary metabolism is presented. It describes the distribution of the carbon-13 isotope (13C) at the different carbon positions of metabolites in cells fed with 13C-enriched substrates. The model allows the determination of fluxes through different metabolic pathways from 13C- and 1H-NMR spectroscopy and mass spectrometry data. The considered metabolic network includes glycolysis, gluconeogenesis, the citric acid cycle and a number of reactions corresponding to protein or fatty acid metabolism. The model was used for calculating metabolic fluxes in a rat tumor cell line, the C6 glioma, incubated with [1-13C]glucose. After evolution to metabolic and isotopic steady states, the intracellular metabolites were extracted with perchloric acid. The specific enrichments of glutamate, aspartate and alanine carbons were determined from 13C-, 1H-NMR spectroscopy, or mass spectrometry data. Taking into account the rate of glucose consumption and of lactate formation, determined from the evolution of glucose and lactate contents in the cell medium, and knowing the activity of the hexose monophosphate shunt, it was possible to estimate the absolute values of all the considered fluxes. From the analysis the following results were obtained. (a) Glucose accounts for about 78% of the pyruvate and 57% of the CoASAc. (b) A metabolic channelling occurs at the citric acid cycle level; it favours the conversion of carbons 2, 3, 4, and 5 of 2-oxoglutarate into carbons 1, 2, 3, and 4 of oxaloacetate, respectively. The percentage of channelled metabolites amounts to 39%. (c) The pyruvate carboxylase activity and the efflux from the citric acid cycle are estimated to be very low, suggesting a lack of glutamine production in C6 cells. The results emphasize different metabolic characteristics of C6 cells when compared to astrocytes, their normal counterpart.