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. Their reversal potentials are presumed to equal the equilibrium potential for Cl- (37). Mechanisms underlying activity-dependent increases in the intracellular concentration of Cl- ([Cl-]i) were investigated by describing active and passive pathways for Cl- influx and efflux. 2. During 99-s applications of GABA, driving force declined by 51% due to increases in [Cl-]i; thus passive Cl- influx through GABA-activated pathways can significantly affect [Cl-]i. 3. Decreasing the extracellular K+ concentration ([K+]o) from 5.8 to 1 mM caused a rapid hyperpolarizing shift in the mean IPSC reversal potential (EIPSC) from -67.6 to -81.9 mV, even when membrane potential (Vm) was maintained constant and depolarized with respect to EIPSC. 4. Decreasing [K+]o from 5.8 to 1 mM caused a rapid hyperpolarizing shift in the mean GABA reversal potential (EGABA) from -64.7 to -81.1 mV, even when Vm was maintained constant and depolarized with respect to EGABA. Reducing the extracellular Cl- concentration from 153 to 89 mM, while maintaining [K+]o constant at 1 mM, shifted the mean EGABA from -81.1 to -66.2 mV, an amount close to that predicted by the Nernst equation for Cl-. We conclude that reducing [K+]o caused a hyperpolarizing shift in EGABA and EIPSC by decreasing [Cl-]i. 5. The shift of EIPSC and EGABA upon alteration of [K+]o did not result from contamination of the responses by additional K+-mediated components because it was unaffected by block of K+ channels with intracellular Cs+. 6. Reducing the extracellular Na+ concentration from 141 to 70 mM had no effect on EGABA. 7. Furosemide, bath-applied at 5 X 10(-4) M while holding Vm depolarized with respect to EIPSC, caused a rapid, reversible decrease in IPSC driving force averaging 69%, consistent with the presence of a furosemide-sensitive outward Cl- -transport system. 8. Reducing [K+]o from 5.8 to 1 mM in the presence of 5 X 10(-4) M furosemide produced a smaller shift of EIPSC from -61.0 to -71.2 mV, however, after washout of furosemide from [K+]o = 1 mM saline, EIPSC shifted further to -89.8 mV.(ABSTRACT TRUNCATED AT 400 WORDS)