5-Hydroxytryptamine (5-HT, also commonly known as serotonin) is synthesized in enterochromaffin cells in the intestine and in serotonergic nerve terminals. In the periphery, 5-HT mediates gastrointestinal motility, platelet aggregation, and contraction of blood vessels. Many functions of the central nervous system are influenced by 5-HT, including sleep, motor activity, sensory perception, arousal and appetite. A family of 12 GPCRs and one ion channel mediate the biological effects of 5-HT. 5-HT4 comprises at least 8 isoforms varying at the C-terminus, which are generated by alternative splicing. The expression and distribution of these splice variants differs among organs and tissues with many of them present in several tissues such as atrium, brain, and GI tract. To date, all isoforms have been shown to activate adenylyl cyclase in vitro, and no difference in signal transduction between C-terminal 5-HT4 receptor variants has been demonstrated. 5-HT4 receptors are of potential interest for the treatment of patients with GI motility disorders and Alzheimer"s disease.