Using very low concentrations (1 mumol range) of L-2-3-[3H]glutamate, (3H-Glu) or L-2-3-[3H]glutamine (3H-Gln), we have previously shown by autoradiography that these amino acids were preferentially taken up in the molecular layer of the cerebellar cortex. Furthermore, the accumulation of 3H-Glu was essentially glial in these conditions. We report here experiments in which uptake and metabolism of either (3H-Glu) or (3H-Gln) were studied in adult rat cerebellar slices. Both amino acids were rapidly converted into other metabolic compounds: after seven minutes of incubation in the presence of exogenous 3H-Glu, 70% of the tissue accumulated radioactivity was found to be in compounds other than glutamate. The main metabolites were Gln (42%), alpha-ketoglutarate (25%) and GABA (1,4%). In the presence of exogenous 3H-Gln the rate of metabolism was slightly slower (50% after seven minutes of incubation) and the metabolites were also Glu (29%), alpha-ketoglutarate (15%) and GABA (5%). Using depolarizing conditions (56 mM KCl) with either exogenous 3H-Glu or 3H-Gln, the radioactivity was preferentially accumulated in glutamate compared to control. From these results we conclude: i) there are two cellular compartments for the neurotransmission-glutamate-glutamine cycle; one is glial, the other neuronal; ii) these two cellular compartments contain both Gln and Glu; iii) transmitter glutamate is always in equilibrium with the so-called "metabolic" pool of glutamate; iv) the regulation of the glutamate-glutamine cycle occurs at least at two different levels: the uptake of glutamate and the enzymatic activity of the neuronal glutaminase.