Motilin is a 22 amino acid polypeptide hormone, secreted by endocrine M cells that are numerous in crypts of the small intestine, especially in the duodenum and jejunum. Based on amino acid sequence, motilin is unrelated to other hormones. Because of its ability to stimulate gastric activity, it was named "Motilin". Apart from in humans, motilin receptors are found in the gastrointestinal tracts of pigs, rats, cows and cats and in the central nervous system of rabbits.

Motilin was discovered by Brown when he introduced alkaline solution into duodenum of dogs, which caused strong gastric contractions. Motilin has 22 amino acids and molecular weight of 2698. In extract from human gut and plasma, there are two basic forms of motilin. The first molecular form is the polypeptide of 22 amino acids.