Many neuropeptides have recently been identified in human and animal airways. These peptides, which may coexist with classical transmitters, have potent effects on airway calibre, blood vessels and secretions, raising the possibility that they may be involved in airway diseases such as asthma. Vasoactive intestinal peptide and peptide histidine methionine have potent relaxant effects on both vascular and bronchial smooth muscle, and may be neurotransmitters of non-cholinergic vasodilatation and non-adrenergic bronchodilation. Several neuropeptides which are found in sensory nerves, such as substance P, neurokinin A and calcitonin gene-related peptide, have both direct inflammatory effects and influence inflammatory cells, and might also contribute to the pathology of asthma if released from sensory nerve endings by an axon reflex.