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1. The inhibitory neuromuscular junction of the abductor muscle of the large claw of the hermit crab (Eupagurus bernhardus) was investigated using electrophysiological intracellular techniques in order to elucidate further the relative contributions of the pre- and post-synaptic mechanisms of action of GABA and of neural inhibition.

2. The electrical constants of the post-synaptic membrane, calculated using the equations for a `short cable' model, were characteristic of a poorly developed electrical excitability; the specific membrane resistance was usually < 1000 cm² and the specific membrane capacitance was > 40 µF/cm².

3. Stimulation of the excitatory axon to the abductor muscle of the large claw at a frequency of 20 Hz evoked highly facilitating excitatory junction potentials (e.j.p.s); stimulation of the inhibitory axon (60-220 Hz) during the excitatory train elicited inhibition which was manifest as an attenuation of the e.j.p.s.

4. The addition of -aminobutyric acid (GABA) to the bathing solution produced a dose-dependent reduction of e.j.p. amplitude and membrane resistance. The inhibitory effect of concentrations (5 x 10^-5 and 1 x 10^-4 M) which caused a 40-75% e.j.p. attenuation could largely be accounted for by a post-synaptic action on membrane conductance.

5. Experiments with picrotoxin suggest that presynaptic inhibitory mechanisms have an important role in neurally evoked inhibition.

6. Picrotoxin (1-5 x 10^-5 M) effectively blocked neural inhibition and the actions of GABA in this preparation, whereas bicuculline proved to be considerably less potent and therefore less useful as a physiological tool for studying GABA-mediated inhibition in crustacea.