This study used in vivo intracellular recording in rat hippocampus to evaluate the effect of lidocaine and MK-801 on the membrane properties and the synaptic responses of individual neurons to electrical stimulation of the commissural pathway. Cells in control group typically fired in a tonic discharge mode with an average firing frequency of 2.4⁑0.9 Hz. Neuron in MK-801 treated group (0.2 mg/kg, i.p.) had an average input resistance of 32.8⁑5.7 MΩ and a membrane time constant of 7.4⁑1.8 ms. These neurons exhibited 2.4⁑0.2 ms spike durations, which were similar to the average spike duration recorded in the neurons of the control group. Slightly less than half of these neurons were firing spontaneously with an average discharge rate of 2.4⁑1.1 Hz. The average peak amplitude of the AHP following the spikes in these groups was 7.4⁑0.6 mV with respect to the resting membrane potential. Cells in MK-801 and lidocaine treated group (5 mg/kg, i.c.v.) had an average input resistance of 34.5⁑6.0 MΩ and an average time constant of 8.0⁑1.4 ms. The cells were firing spontaneously at an average discharge rate of 0.6⁑0.4 Hz. Upon depolarization of the membrane by 0.8 nA for 400 ms, all of the tested cells exhibited accommodation of spike discharge. The most common synaptic response contained an EPSP followed by early-IPSP and late-IPSP. Analysis of the voltage dependence revealed that the early-IPSP and late-IPSP were putative Cl⁣-and K⁢-dependent, respectively. Systemic injection of the NMDA receptor blocker, MK-801, did not block synaptic responses to the stimulation of the commissural pathway. No significant modifications of EPSP, early-IPSP, or late-IPSP components were detected in the MK-801 and/or lidocaine treated group. These results suggest that MK-801 and lidocaine manifest their CNS effects through firing pattern of hippocampal pyramidal cells and neural network pattern by changing the synaptic efficacy and cellular membrane properties.