The aim of this paper is to clarify the mechanism through which the taurine analogue guanidinoethane sulfonate (GES) produces its epileptogenic effects. Experiments were performed in the rat hippocampus in vivo, using a brain dialysis probe also containing a recording electrode. Perfusion of 10 mM GES induced an enhancement of extracellular taurine levels probably as a result of forced efflux through the taurine uptake systems in a heteroexchange process. This taurine increase was highly reversible. GES also induced an increase of neuronal excitability and an impairment of recurrent inhibition as judged by the neuronal pattern discharge of evoked potentials. These results indicate the possible implication of GABA receptors in the epileptogenic effect of GES. Specific binding of [3H]-GABA to P2 fractions was inhibited by both bicuculline methiodide (BMI) and GES with the same potency. Similar results were obtained using cerebral sections. Autoradiographic experiments confirm the binding results. GES and BMI completely displaced [3H]-GABA binding. All these results suggest that the epileptogenic GES action is due to a direct antagonism on GABAA receptors.