Abstract
1. Excitatory postsynaptic potentials (e. p.s. p.'s) of the giant nerve cell system of Onchidium verruculatum were investigated by intracellular electrode technique.
2. An impulse of a single presynaptic fibre was usually too small to initiate a postsynaptic spike and spacial and temporal summation was necessary to produce the postsynaptic spike.
3. The increase of calcium or magnesium concentration in the external medium increased or decreased the amplitude of the e. p.s. p.respectively but the maximum amplitude of the synaptic potential reached by repetitive presynaptic stimulation was practically unchanged.
4. From the above results the temporal facilitation is produced by increasing the active area of the synaptic membrane innervated by a single presynaptic fibre.
