When the innervated membrane of the electric eel [Electrophorous electricus] electroplaque is exposed to concentrations of .DELTA.9-tetracannabinol [THC] of 50-100 .mu.M synaptic transmission is blocked in approximately 30 min. During the block, the chemosensitivity of the innervated membrane to cholinergic agonists remains unaltered. Direct stimulation of the innervated membrane after synaptic transmission had failed did not elicit any postsynaptic membrane action potentials. The passing of hyperpolarizing currents across the electroplaque caused the nerve terminals which innervate the electroplaque to produce subthreshold potentials which resemble endplate potentials with a prolonged falling phase. With further incubation in the THC solution the subthreshold potentials decreased in size and eventually disappeared. These results indicate that .DELTA.9-THC does not affect the acetylcholine-mediated membrane permeability changes, but instead, reduces the excitability of the nonsynaptic membrane of both the pre- and postsynaptic elements.