The membrane of small crayfish muscle fibers was clamped to potentials between-150 and -20 mV and amplitude and time course of inhibitory postsynaptic currents (IPSCs) were studied. The IPSCs were recorded extracellularly by means of a focal microelectrode and also as total clamp current. The IPSCs lasted about 40 ms and were slowed by depolarization. The rate constant alpha of decay of the IPSC depended on membrane potential E according to the relation alpha= 44 s-1 -e-8.7 V E at 13.5 degrees C. alpha increased with temperature with a Q10 of 1.9 to 2.5. The amplitude iI of the IPSC depended nonlinearly on E and decreased with time after a potential shift. This was partly due to movement of Cl--ions, the difference (E--EI) between clamp potential and reversal potential for the IPSC decreasing to a few mV within several minutes after a shift in E. The inhibitory conductance gI increased up to 30-fold for 100 mV depolarization also changed with time. However, the the inhibitory permeability PI proved to be independent of membrane potential and time. The potential dependence of gI is thus largely due to changes in the internal Cl--concentration.