The 5-HT(3) receptor is a ligand-gated ion channel activated by serotonin (5-HT). Although originally identified in the peripheral nervous system, the 5-HT(3) receptor is also ubiquitously expressed in the central nervous system. Sites of expression include several brain stem nuclei and higher cortical areas such as the amygdala, hippocampus, and cortex. On the subcellular level, both presynaptic and postsynaptic 5-HT(3) receptors can be found. Presynaptic 5-HT(3) receptors are involved in mediating or modulating neurotransmitter release. Postsynaptic 5-HT(3) receptors are preferentially expressed on interneurons. In view of this specific expression pattern and of the well-established role of 5-HT as a neurotransmitter shaping development, we speculate that 5-HT(3) receptors play a role in the formation and function of cortical circuits.