The effect of L-cysteine sulfinic acid (CSA) and L-homocysteic acid (HCA) on the release of tritiated gamma-amino butyric acid ([3H]GABA), from the external plexiform layer (EPL) of the rat olfactory bulb, was compared with that of glutamate. These amino acids induced release of GABA was strongly inhibited by the glutamate uptake blocker, pyrrolidine-2,4-dicarboxylate (2,4,PDC) (50 microM), while it was not inhibited by the specific GABA uptake blockers nipecotic acid (0.5 mM) or NO-711 (5 microM). Only the HCA induced GABA release was 60% inhibited by beta-alanine (0.5 mM), a glial GABA uptake blocker and 78% by the NMDA receptor antagonist 2-amino-5-phosphonopentanoic acid (AP-5) (100 microM). The non-NMDA receptor antagonists 6-cyano-2,3-dihydroxy-7-nitro-quinoxaline (CNQX) up to 500 microM had no effect on HCA or CSA stimulated GABA release. These results bring evidence for an excitatory role of HCA and CSA together with glutamate on GABAergic neuronal or glial elements, in the olfactory bulb. This role could be mediated through the reversal of the glutamate or/and the glial GABA transporter and through the activation of a NMDA type receptor.