GABA A- and GABA B-receptor-specific agonists inhibit the depolarization-evoked release of acetylcholine in cholinergic synaptosomes from Torpedo electric organ. Over 60% of the release is inhibited by a 10 −4 M concentration of GABA itself. ic 50 s for muscimol and baclofen are1.3 × 10 −4 and2.2 × 10 −6M, respectively. The effect of muscimol is totally blocked by the direct antagonist bicuculline methiodide, and also by the allosteric antagonists methyl 6,7-dimethoxy-4-ethyl-β-carboline-3-car☐ylate, picrotoxinin and tert-butylbicyclo-orthobenzoate; the effect of baclofen is blocked by δ-aminovalerate. Furthermore, the inhibitory action of muscimol on acetylcholine release is substantially enhanced by flunitrazepam and pentobarbital. These results suggest the existence of typical GABA A and GABA B receptors in the presynaptic nerve terminals of the Torpedo electric organ regulating the liberation of acetylcholine and therefore the discharge of the electroplaques.