1. Mechanisms of depolarizing synaptic inhibition were investigated at the crayfish giant motor synapse with the use of two-electrode current- and voltage-clamp techniques. Depolarizing inhibitory postsynaptic potentials (d-IPSPs) of between 5 and 15 mV in amplitude are produced there in the motor giant motoneuron (MoG) by motor giant inhibitor (MoGI) interneurons. 2. Three mechanisms of inhibition are activated by the d-IPSP: inactivation of a voltage-sensitive inward current (probably sodium), activation of the delayed rectifier, and reverse bias of the electrically rectifying giant motor synapse (GMS). These mechanisms supplement the inhibition produced by a gamma-aminobutyric acid (GABA)-mediated increase in postsynaptic conductance. 3. The d-IPSP is produced by a fast-rising increase in postsynaptic membrane conductance that peaks at 10 microS and lasts nearly 100 ms. 4. An 8-ms, 10-mV depolarizing prepulse inactivated 90% of the inward current evoked by a subsequent step to 33 mV above rest potential, which was -70 mV. d-IPSPs having similar amplitudes should have similar effects on the inward current evoked by an excitatory postsynaptic potential (EPSP). 5. The input resistance of MoG decreased by greater than 60% when the cell was depolarized to 11 mV above rest. This resistance change corresponds to delayed rectification, which should also contribute to the increase in input conductance during a d-IPSP. 6. Depolarization of MoG by 10 mV reduced the excitatory postsynaptic current through the GMS by up to 30%. The reduction in synaptic current occurs because postsynaptic depolarization reduces the transynaptic driving force and increases the reverse bias of the electrically rectifying synapse.(ABSTRACT TRUNCATED AT 250 WORDS)