[en] This report describes the distribution and nature of lymphoid tissue in the nasopharyngeal mucosa of six puppies (mean age +/- SD, 0.3 +/- 0.25 years) and eight adult dogs (mean age +/- SD, 8.8 +/- 2.67 years) without respiratory disease. A non-encapsulated area of organized mucosa-associated lymphoid tissue was observed in the caudal part of the posterior wall of the nasopharynx, distal to the openings of the auditory tubes. This structure was consistent with the pharyngeal tonsil and was microscopically more extensive in puppies than in adult dogs. Histochemistry and immunohistochemistry were used to characterize and enumerate the leucocyte subsets in this part of the nasopharynx. Mast cells were found immediately beneath the respiratory epithelium but were also scattered in the glandular and muscular tissue. IgA(+) plasma cells outnumbered IgG(+) and IgM(+) plasma cells, especially in the glandular tissue. All classes of plasma cells were present in significantly greater numbers in adults than in puppies. MHC class II+ cells were mainly observed in areas containing diffuse and follicular aggregates of lymphoid cells. Both MHC class II+ cells and CD1c(+) cells with a dendritic morphology were predominantly found immediately beneath or within the epithelium, and cells expressing these markers were more abundant in puppies than in adult dogs. The anti-L1 marker labelled low numbers of cells with a neutrophilic morphology, which were significantly more abundant in puppies than in adult dogs. The majority of lymphoid cells were CD3(+) T lymphocytes and these were particularly abundant in areas containing aggregates of lymphold cells; CD4(+), CD8(+) and TCR alpha beta(+) cells had the same distribution as the CD3(+) cells. CD4(+) cells were more numerous than CD8(+) cells. The quantitative and qualitative data obtained will enable comparisons to be made with similar studies in dogs suffering from nasopharyngeal diseases, or when the local immune system needs to be investigated. (c) 2006 Elsevier Ltd. All rights reserved.