Intracellular recording and injection of horseradish peroxidase in rat hippocampal slice preparations revealed the existence of two physiologically distinct types of non-pyramidal cells: one a fast spiking type and the other a non-fast spiking type. In the CAl pyramidal cell layer and the subgranular zone of the dentate gyrus fast spiking cells were found, while in the border area between the stratum lacunosum-moleculare and the stratum radiatum, non-fast spiking cells were found exclusively. These two types of non-pyramidal cells may play different roles in the functions of hippocampal neuronal networks.