Abstract l-α-Aminoadipic acid is a lysine metabolite with neuroexcitatory properties, and has previously been shown to inhibit the production of the broad spectrum excitatory amino acid receptor antagonist kynurenic acid in brain tissue slices. The effects of l-α-aminoadipic acid on the levels of extracellular kynurenic acid were now studied by microdialysis in the dorsal hippocampus of freely moving rats. Application of l-α-aminoadipic acid through the microdialysis probe dose dependently decreased both the concentration of endogenous kynurenic acid and of kynurenic acid which was produced de novo from its bioprecursor l-kynurenine (500 μM applied through the probe). 500 μM l-α-aminoadipic acid lowered the kynurenic acid concentration in the dialysate by 47% and 28% with and without precursor loading, respectively, whereas d-α-aminoadipic acid was without effect. Co-administration of 500 μM l-α-aminoadipic acid with 50 μM veratridine, which by itself produces a substantial decrease in the levels of extracellular kynurenic acid, did not result in a further reduction in kynurenic acid concentrations. Extensive neuronal degeneration caused by an intrahippocampal injection of quinolinic acid (120 nmol) did not interfere with the effect of l-α-aminoadipic acid. Taken together, these data suggest that the effect of l-α-aminoadipic acid on extracellular kynurenic acid levels is likely due to its direct action on astrocytes, which are known to harbor kynurenic acid's biosynthetic enzyme, kynurenine aminotransferase. l-α-Aminoadipic acid may modulate kynurenic acid function in the brain and thus play a role in the pathogenesis of neurodegenerative and seizure disorders.