Mice were injected intravenously with [2-13C]-acetate or [1-13C]glucose and killed after 5, 15, or 30 min. Another group of animals was injected three times subcutaneously during 30 min with [2-13C]acetate to achieve a steady-state-like situation. Brain extracts were analyzed by 13C NMR spectroscopy, and the percent enrichment of various carbon positions was calculated for amino acids, lactate, and glucose. Results obtained with [2-13C]acetate, which is metabolized by glia and not by neurons, showed that glutamine originated from a glial tricarboxylic acid cycle (TCA cycle) that loses 65% of its intermediates per turn of the cycle. This TCA cycle was associated with pyruvate carboxylation, which may replenish virtually all of this loss, as seen from the labeling of glutamine from [1-13C]glucose. From the C-3/C-4 labeling ratios in glutamine and glutamate and from the corresponding C-3/C-2 labeling ratio in GABA obtained with [2-13C]acetate, it was concluded that the carbon skeleton of glutamine to some extent was passed through TCA cycles before glutamate and GABA were formed. Thus, astrocytically derived glutamine is not only a precursor for transmitter amino acids but is also an energy substrate for neurons in vivo. Furthermore, the neuronal TCA cycles may be control points in the synthesis of transmitter amino acids. Injection of [2-13C]acetate led to a higher 13C enrichment of the C-2 in glutamate and of the corresponding C-4 in GABA than in the C-3 of either compound. This could reflect cleavage of [2-13C]-citrate and formation of [3-13C]oxaloacetate and acetyl-Coa, i.e., the first step in fatty acid synthesis. [3-13C]-Oxaloacetate would, after entry into a TCA cycle, give the observed labeling of glutamate and GABA.