5-Hydroxytryptamine (5-HT) released from enterochromaffin cells regulates gastrointestinal function in either an excitatory or inhibitory manner. 5-HT(3) and 5-HT(4) 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-HT(4) receptors. 5-HT(4) receptor agonists can stimulate upper or lower gut motility, depending on their selectivity and affinity. In the guinea pig colon, the distribution of 5-HT(4) 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-HT(4) receptors. In the light of these findings on the distribution of 5-HT(4) receptors, the effects of receptor agonist compounds could vary depending on the species of experimental animal and the anatomical region studied.