Nerve fibers containing neuropeptide Y, vasoactive intestinal polypeptide/peptide histidine isoleucine, substance P/neurokinin A (NKA), calcitonin-gene-related peptide (CGRP), and galanin are numerous in the respiratory tract. The fibers derive from several different ganglia such as the superior cervical and stellate ganglia, the jugular-nodose ganglionic complex, small ganglia along the vagus nerve, local ganglionic formations in the tracheal wall, and dorsal root ganglia at the cervical and thoracic level. The fibers surround vascular and nonvascular smooth muscle and seromucous glands and can be seen beneath the surface epithelium. In addition, nerve fibers containing substance P/NKA and CGRP penetrate into the surface epithelium. In some instances, neuropeptides are colocalized with "classic" neurotransmitters such as norepinephrine and acetylcholine; in other cases, several peptides coexist. Many of the neuropeptides affect blood flow, smooth muscle tone, and/or seromucous secretion. Some of them may take part in reflex-mediated responses. However, the lack of specific antagonists makes it difficult to define their functional role in the respiratory tract.