Inhibitory postsynaptic potentials (IPSPs) recorded at room temperature in cultured rat hippocampal neurons had the same reversal potential as Cl--dependent voltage responses to gamma-aminobutyric acid (GABA). The IPSPs had a relatively short latency and long duration and could be evoked for hours without change in their properties. They were consistently depressed by picrotoxin applied near cell bodies of the neurons under study. Postsynaptic cells exhibiting IPSPs were voltage clamped with two electrodes for the purpose of studying the properties of evoked inhibitory postsynaptic currents (IPSCs). The IPSCs shared the same reversal potential and sensitivity to [Cl-]i as was observed with membrane current responses to GABA. They were both depressed by picrotoxin, with little if any change in the kinetics of ion channel activity either estimated from fluctuation analysis of drug-depressed current responses to GABA or calculated from semilogarithmic plots of IPSC decay. The decay of the IPSC was well fitted by a single exponential with a time constant of about 20 ms, which corresponded closely to the estimated average duration of an ion channel activated by GABA. IPSC decay was sensitive to the potential at which the cell was held, increasing by up to 50% in some cells clamped at positive potentials relative to values obtained at the level of the resting potential. IPSCs were enhanced in amplitude by diazepam, which also prolonged their time constant of decay. Diazepam potentiated membrane current responses to GABA and fluctuation analysis of potentiated responses indicated that the drug effects could be accounted for by an increase both in estimated channel duration and channel frequency. IPSCs were also altered by pentobarbital, which markedly prolonged their time constant of decay with little if any change in their amplitude. Pentobarbital enhanced current responses to GABA, an effect that could be accounted for primarily in terms of a pronounced increase in estimated channel lifetime. None of the drugs used in the present study affected the elementary conductance estimated from fluctuation analysis of GABA-evoked current response. The results suggest that IPSPs and IPSCs evoked in these cultured hippocampal cells are mediated by GABA, about 1,700 Cl- ion channels are activated at the peak of the synaptic conductance, and clinically important drugs act postsynaptically on the kinetics of the channels so as to change the amplitude and/or time course of the synaptic conductance.