Leucine is rapidly metabolized in astroglial primary cultures. Therefore, it is considered as valuable fuel in brain energy metabolism. Only few of the leucine metabolites generated and released by astroglial cells have been identified. Therefore, a more detailed study was performed analyzing by NMR techniques the 13C-labeled metabolites, which were released by astroglial primary cultures during the degradation of [U-(13)C]leucine. Confirming a former radioactive study this analysis revealed 13C-labeled 2-oxoisocaproate and ketone bodies. Additionally, 13C-labeled alanine, citrate, glutamine, lactate and succinate were identified. Their detailed isotopomer analysis proves that 13C-labeled acetyl-CoA enters the tricarboxylic acid cycle, that intermediates with a characteristic 13C-labeling pattern can be withdrawn at several positions of the cycle, and that in the case of lactate and alanine there appears to be a participation of an active phosphoenolpyruvate carboxykinase and/or malic enzyme pathway. Thus, astroglial cells generate and release into the extracellular fluid not only the leucine catabolites 2-oxoisocaproate and ketone bodies, but also several tricarboxylic acid cycle dependent metabolites.