Peptides abound in the enteric nervous system where they coexist with other peptides and with other neurotransmitter substances. Each functional class of enteric neuron contains unique combination of chemicals, including peptides. About twenty functional classes of enteric neurons have been identified using such 'chemical coding' principles. Peptides play multiple roles in the neural control of gastrointestinal functions depending on the class of neurons in which they are contained and on the relative importance in the multiple mechanisms of neurotransmission. Peptides are involved in motility, mucosa transport, and circulation and also in growth, morphogenesis defense, and immune functions of the gut.