The present paper describes a new planar multielectrode array (the MED probe) and its electronics (the MED system) which perform electrophysiological studies on acute hippocampal slices. The MED probe has 64 planar microelectrodes, is covered with a non-toxic, uniform insulation layer, and is further coated with polyethylenimine and serum. The MED probe is shown to be appropriate for both stimulation and recording. In particular, multi-channel recordings of field EPSPs obtained by stimulating with a pair of planar microelectrodes were established for rat hippocampal acute slices. The recordings were stable for 6 h. Finally a spatial distribution of long-term potentiation was studied using the MED system.