The occurrence and distribution of nerve fibres containing gastrin-releasing peptide (GRP) were investigated in the respiratory tract of several mammals using immunocytochemistry. A moderate supply of nerve fibres displaying GRP immunoreactivity was seen in the middle ear mucosa, the nasal mucosa and the tracheobronchial wall. Generally, the fibres were distributed around blood vessels and seromucous glands. In addition, scattered GRP fibres were seen in the smooth muscle of the tracheal wall. The distribution of GRP fibres in the respiratory tract suggests multiple functions of GRP such as regulation of local blood flow, glandular secretion and smooth muscle activity.