1. Single-electrode voltage-clamp recordings were made from CA3 pyramidal cells in organotypic hippocampal slice cultures for measurement of membrane currents underlying both the gamma-aminobutyric acid (GABA)-mediated, Cl- -dependent inhibitory postsynaptic potential (IPSC), evoked in response to stimulation of the mossy fiber pathway, and responses to iontophoretically applied GABA. Pre- and postsynaptic mechanisms mediating activity-dependent reductions in the conductance underlying the IPSC (gIPSC) were investigated. 2. During 99-s applications of GABA, the mean evoked conductance (gGABA) decreased 43% with an initial time constant of 51 s. Desensitization was never complete. 3. Ca2+-influx, activated with depolarizing voltage commands of 100-ms to 15-s duration in the presence of intracellular Cs+, had no effect on GABA responses. 4. Iontophoretic application of the GABAA-receptor agonist muscimol caused a rapid decrease of 80-100% in the amplitude of IPSCs evoked at depolarized membrane potentials (Vm). Recovery was 80% complete in 30 s. The second of two paired applications of muscimol, delivered at the same iontophoretic intensity, was reduced in amplitude 35%. This was shown to result from a decrease in driving force rather than from desensitization. We conclude that muscimol decreases IPSCs by causing an increase in the intracellular Cl- concentration. 5. Iontophoretic application of the GABAB-receptor agonist (+/-)-baclofen caused a decrease of only 30% in the amplitude of IPSCs evoked at depolarized Vms. This effect outlasted the post-synaptic effects of baclofen; recovery was 80% complete between 60 and 90 s. 6. Bath application of (-)-baclofen was found to decrease gIPSC without affecting the IPSC reversal potential. This effect was rapid in onset, could be observed at concentrations as low as 1 X 10(-7) M, and recovered quickly. The EC50 was roughly 5 X 10(-7) M and appeared similar to that for the baclofen-activated increase in postsynaptic conductance. No effect on responses to iontophoretically applied GABA was observed, demonstrating that baclofen decreases gIPSC by reducing presynaptic release via GABAB receptors. 7. Iontophoretic application of GABA reduced IPSCs in a dose-dependent manner. At low iontophoretic intensities, IPSCs were reduced only 30% and recovered slowly, as with baclofen iontophoresis. At higher iontophoretic intensities, IPSCs were more completely blocked. Recovery was initially fast, but took 60-90 s to be complete.(ABSTRACT TRUNCATED AT 400 WORDS)