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1. In Aplysia fasciata, intrasomatic stimulation of the giant cell (RGC) of the right upper quadrant of the abdominal ganglia is followed after a constant delay by the appearance of a synaptic potential recorded in the giant cell (LGC) of the left pleural ganglion.

2. The synaptic potential recorded in the LGC soma has a biphasic form (hence biphasic post-synaptic potential or BPSP) consisting of a fast depolarizing phase of about 200-800 µV amplitude and 0·15-0·25 sec duration, followed by a slow hyperpolarizing phase of about 200-800 µV amplitude and 1-3 sec duration.

3. During repetitive stimulation summation results in an over-all hyperpolarization at low frequencies (less than 5/sec) and an over-all depolarization for higher frequencies. Very high frequencies (25/sec) of stimulation of the RGC may elicit a spike in the LGC.

4. Stimulation with two shocks showed increasing effects with shorter intervals on both the depolarizing and hyperpolarizing phase. These effects were progressive and there was no falling out of one of the two phases as might be expected if the BPSP was a composite of an inhibitory post-synaptic potential (IPSP) and an excitatory post-synaptic potential (EPSP).

5. The effects of artificially imposed polarization of the LGC through a second micro-electrode suggest that the BPSP results from a chemical transmission mechanism for both its depolarizing and hyperpolarizing phases but electrical transmission cannot be excluded.

6. Curare has no effect on the BPSP and thus excludes a cholinergic transmission mechanism. Chloride ions injected into the LGC soma do not appear to modify the BPSP and hence it is concluded that the hyperpolarizing phase is different from IPSPs of the same cell.

7. No synaptic potential is recorded in the RGC following stimulation of the LGC, except in a single preparation in which the RGC soma was situated in the right pleural ganglion. In this case the synaptic potential recorded in both giant cells following stimulation of the other, was biphasic in form.

8. It is concluded that the BPSP is a unitary monosynaptic potential which is a characteristic feature of the organization of these two giant cells.