Serotonin exerts multiple electrophysiological effects on neurons of the central nervous system. It is now known that this diversity reflects at least in part the existence of multiple serotonin receptor subtypes. An example of this occurs in the CA1 region of the hippocampus where as many as ten different serotonin receptor subtypes appear to be expressed. Recent electrophysiological studies have been able to assign specific functional roles to at least 5 of these receptors. These receptors are differentially expressed in the two different cell types present in this region, pyramidal cells and GABAergic interneurons, and mediate different effects on membrane excitability. This distribution is consistent with the different functional roles played by these cells in hippocampus. Thus the differential expression of serotonin receptor subtypes in the CA1 region allows serotonin to modify the function of hippocampal neuronal networks in a manner that is both selective and precise.