Abstract 1. 1. Intracellular recordings were made from the body musculature of the leech Hirudo medicinalis. Resting potential values ranges from −22 to −64 mV. Excitatory potentials ranged in amplitude from 100 μV to 30 mV. 2. 2. Atropine, benzoquinonium, decamethodium, gallamine, hexamethonium and tubocurarine were applied to the excitatory potentials by means of difusion electrodes with tip diameters of between 2 and 10 μ. 3. 3. Benzoquinonium, decamethonium, gllamine and tubocurarine all decreased the amplitude fo the spontaneous excitatory potentials without altering their frequency. 4. 4. It is concluded that all four antagonists exerted their action mainly postsynaptically on the muscle.