The role of serotonin as a neurotransmitter in the vertebrate central and peripheral nervous systems has been extensively studied. In addition to its well-defined role in neurotransmission, serotonin has also been implicated in development. We have used in situ hybridization to localize 5-HT3 receptor mRNA in embryonic rat sections from Embryonic Day 10 (E10) to E18. Expression was first detected in the cranial sensory ganglia starting on E10. Expression was later detected in many regions within the developing CNS. In addition to the cranial sensory ganglia, expression was detected in many regions of the peripheral nervous system including dorsal root, sympathetic, and parasympathetic ganglia, and in the myenteric plexus of the enteric nervous system. Expression was also detected in many nonneuronal cell populations including the choroid plexus, cochlear duct, and olfactory epithelium. Expression in regions of active epithelial-mesenchymal interaction such as the tooth bud, lung, and submandibular gland may indicate a role for 5-HT3 receptors in the process of secondary induction.