VIP is a peptide of 28 amino acid residues that belongs to a glucagon/secretin superfamily, the ligand of class II G protein–coupled receptors.

[5] VIP is produced in many tissues of vertebrates including the gut, pancreas, cortex, and suprachiasmatic nuclei of the hypothalamus in the brain.

[6][7][8] VIP stimulates contractility in the heart, causes vasodilation, increases glycogenolysis, lowers arterial blood pressure and relaxes the smooth muscle of trachea, stomach and gallbladder.

[10] In the digestive system, VIP seems to induce smooth muscle relaxation (lower esophageal sphincter, stomach, gallbladder), stimulate secretion of water into pancreatic juice and bile, and cause inhibition of gastric acid secretion and absorption from the intestinal lumen.

[16][17] VIP is also found in the brain and some autonomic nerves: One region includes a specific area of the suprachiasmatic nuclei (SCN), the location of the 'master circadian pacemaker'.

[22] The SCN coordinates daily timekeeping in the body and VIP plays a key role in communication between individual brain cells within this region.

Combined, these roles in the SCN make VIP a crucial component of the mammalian circadian timekeeping machinery.

Previous pharmacological research has established that VIP is needed for normal light-induced synchronization of the circadian systems.

Application of VIP also phase shifts the circadian rhythm of vasopressin release and neural activity.

Another hypothesis supports VIP sending a paracrine signal from a distance rather than the adjacent postsynaptic neuron.

[18][23] VIP and vasopressin are both important for neurons to relay information to different targets and affect neuroendocrine function.

In certain species of birds with a knockout VIP gene there was an observable decrease in overall aggression over nesting territory.