Helospectin (HS) and pituitary adenylate cyclase activating peptide (PACAP) are newly discovered peptides isolated from the salivary gland venom of the lizard Heloderma horridum and the ovine hypothalamus, respectively. They show chemical similarities to vasoactive intestinal polypeptide (VIP), appear to have similar functions and are present in gut, brain, lung, male and female genitourinary tract. In the present study, the distribution of the helospectin and PACAP-27 in the human upper respiratory system was investigated using indirect immunofluorescence and electron-microscopical ABC-pre-embedding methods. Immunohistochemistry revealed helospectin-like (HS-LI) and PACAP-like (PACAP-LI) immunoreactivity in nerve fibers in human nasal, the larynx (vocal cord, ventricular fold, epiglottis), the tongue and the soft palate mucosa. Helospectin-LI and PACAP-LI containing nerve fibers were mainly found in close association to blood vessels and glandular structures. Colocalization studies carried out by application of double immunofluorescence showed that HS and/(or) PACAP-LI coexist with VIP in apparently the same nerve fibers in the upper respiratory system, although single nerve fibers seem to exclusively express helospectin. The localization patterns of helospectin and PACAP-LI in the human upper respiratory system suggests their possible involvement in the regulation of secretory activities and local blood flow.