The reversal potential for the EPSP in the squid giant synapse has been studied by means of an intracellular, double oil gap technique. This method allows the electrical isolation of a portion of the axon from the rest of the fiber and generates a quasi-isopotential segment. In order to make the input resistance of this nerve segment as constant as possible, the electroresponsive properties of the nerve membrane were blocked by intracellular injection of tetraethylammonium (TEA) and local extracellular application of tetrodotoxin (TTX). Thus, EPSP's could be evoked in the isolated segment with a minimal amount of electroresponsive properties. The reversal potential for the EPSP (EEPSP) was measured by recording the synaptic potential or the synaptic current during voltage clamping. The results indicate that EEPSP may vary from +15 to +25 mV, which is more positive than would be expected for a 1:1 conductance change for Na+ and K+ (approximately -15 mV) and too negative for a pure Na+ conductance (+40 mV). This latter value (ENa) was directly determined in the voltage clamp experiments. The results suggest that the synaptic potential is probably produced by a permeability change to Na+ to K+ in a 4:1 ratio. No change in time-course was observed in the synaptic current at clamp levels of -100 and +90 mV. The implications of a variable ratio for Na+-K+ permeability in subsynaptic-postsynaptic membranes are discussed.

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