5-Hydroxytryptamine (5-HT) released from enterochromaffin cells regulates gastrointestinal function in either an excitatory or inhibitory manner. 5-HT₃ and 5-HT₄ receptors in the gut have been the focus of clinical studies on the management of gastrointestinal motility disorders. 5-HT stimulates intestinal propulsive reflexes through 5-HT₄ receptors. 5-HT₄ receptor agonists can stimulate upper or lower gut motility, depending on their selectivity and affinity. In the guinea pig colon, the distribution of 5-HT₄ receptors in the myenteric plexus and circular muscle layer differs between the proximal and distal regions. 5-HT stimulates intestinal motility via excitatory neurons while causing relaxation of the circular muscle via 5-HT₄ receptors. In the light of these findings on the distribution of 5-HT₄ receptors, the effects of receptor agonist compounds could vary depending on the species of experimental animal and the anatomical region studied.