We have attempted to suppress voltage-dependent conductances in hippocampal neurons by introducing various intracellular agents. Voltage-clamp studies were carried out using acutely dissociated hippocampal neurons from adult guinea pigs. Synaptic events were examined using intracellular recordings in the slice preparation. Sodium conductance was suppressed when the quaternary lidocaine derivative QX 314 was introduced intracellularly. Potassium conductances were blocked by intracellular cesium or Tris. We also found that the anion fluoride could affect calcium conductance by an intracellular action. When anions other than fluoride were used for intracellular recordings, the voltage-dependent calcium current inactivated slowly and showed persistent activation at membrane potentials between -40 and -10 mV. In contrast, when fluoride was present intracellularly, the inactivation kinetics of the calcium current were accelerated and the persistent component of the current was largely suppressed. Intracellular recordings in the hippocampal slice showed that when electrodes contained cesium, QX 314, and fluoride, the spiking and nonlinear responses of the neuronal membrane to depolarization were blocked. In these conditions the time course and voltage-dependence of EPSPs could be examined in detail without complications due to voltage-dependent currents of the postsynaptic cell.