The mucosal membranes form a weak mechanical barrier, but they are provided with an extensive specific and non-specific defence system. Antigenic stimulation of the mucosal immune system of the oronasal passages induces specific, local immune responses, and activates immune components of mucosae elsewhere as well as the systemic immune system. Nasal lymphocytes are disseminated diffusely in the mucosa or are organised in structures at the entrance of the nasopharynx (nasal-associated lymphoid tissues, NALT). Nasal lymphatics, and possibly NALT, play an important role in drainage of brain fluid, especially in small animals. Little is known about toxicity to the NALT, despite its central role in mucosal immunity. Its strategic position in the nasal passages suggests that it comes easily into contact with inhaled nasal toxicants. Therefore, we recommend to include histopathological examination of NALT in standard guideline-driven inhalation toxicity studies. © 2003 Elsevier Science Ireland Ltd. All rights reserved.