The 5-HT4 receptor is a member of the seven transmembrane spanning G-protein-coupled family of receptors. The receptor is positively coupled to adenylate cyclase and exists in two isoforms (5-HT4S and 5-HT4L) that differ in the length and sequence of their carboxy termini. The 5-HT4 receptor is pharmacologically defined by selective agonists such as SC 53116 and RS 67506, and selective antagonists such as GR 113808, SB 204070, and RS 39604. The receptor is widely distributed in the central nervous system and peripheral tissues. In the periphery, the receptor plays an important role in the function of several organ responses including the alimentary tract, urinary bladder, heart and adrenal gland. In the alimentary tract, stimulation of 5-HT4 receptors has a pronounced effect on smooth muscle tone, mucosal electrolyte secretion, and the peristaltic reflex. In the urinary bladder, activation of 5-HT4 receptors modulates cholinergic/purinergic transmission. In the heart, stimulation of atrial 5-HT4 receptors produces positive inotropy and tachycardia that can precipitate arrhythmias. In the adrenal gland, agonism of 5-HT4 receptors stimulates release of cortisol, corticosterone, and aldosterone. Since its discovery in 1988, significant advances have been made in our understanding of the physiology and pharmacology of the 5-HT4 receptor. These advances have led to the development of several selective 5-HT4 receptor agonists and antagonists that may have therapeutic utility in the treatment of peripheral disorders such as irritable bowel syndrome, gastroparesis, urinary incontinence and cardiac arrhythmias.