The ionic mechanism of the inhibitory postsynaptic potential (IPSP) was studied at the giant motor fiber of crayfishProcambarus clarki (Gerard). The reversal potential of the IPSP was −53±5 mV (mean and s.d. from 51 fibers), while the resting potential was −62±6 mV. Cl was thought to be the main ion species responsible for the IPSP because 1) the reversal potential of the IPSP was not changed significantly by K-free solution, 2) replacement of the external Cl with propionate reduced the membrane conductance increase induced by GABA to 10% of the control value and 3) increase of the K concentration from 5.4 to 27 mM in propionate solution did not change the membrane conductance increase induced by GABA. The external Cl was also replaced with Br, I, NO3 and SCN. These ions shifted the reversal potential of the IPSP and the membrane potential in GABA to the negative direction in the order: SCN>I>NO3>Br. It is likely that the external anions influence the passage of the internal anion through the activated inhibitory subsynaptic membrane.