Action potentials intracellularly recorded from near the nerve terminals of the opener excitor motor axon in crayfish are reduced in amplitude during presynaptic inhibition. The amplitude and sign (hyperpolarizing or depolarizing) of presynaptic inhibitor potentials (PIPs) depends upon the relationship between the resting membrane potential of each excitor axon and its PIP equilibrium potential (which equals the equilibrium potential for gamma-aminobutyric acid).