There are many factors contributing to paranasal sinusitis, such as respiratory tract infection, allergy and anatomical conditions of nasal cavity and paranasal sinus, especially natural ostium. Normal sinus ostium is closely related to morphological state of mucociliary system. Stenosis or obstruction of sinus ostium affects the sinus gas composition, pH and viscosity of mucus, and eventually the sinus will be susceptible to infection. The purposes of present study are to investigate morphological changes and secretory function of sinus mucosa simultaneously, and to identify pathophysiological findings of experimentally induced sinusitis in white rabbit. Experimental groups were divided into control, acute and chronic sinusitis group according to experimental period. Partial obstruction of natural ostium was made by Histoacryl(butylcyanocrylate) and injection of 10⁷~10⁹ Streptococcus pneumoniae was done to induce sinusitis experimentally. The results obtained were as follows ; 1) Essential steps for induction of experimental acute and chronic sinusitis in rabbit were partial obstruction of natural ostium and bacterial inoculation. 2) During whole process of paranasal sinusitis, ciliated cells were partialy replaced by non-ciliated undifferentiated cells and goblet cells with abundant secretion. The mucus in goblet cells stained predominantly with alcian blue stain and periodic acid Schiff reactions, and was thus presumed to contain complex glycoconjugate. 3) In acute paranasal sinusitis, the number of ciliated cells was decreased but that of non-ciliated cells was increased. Vacuolization within cell, emergence of undifferentiated cell, and ciliogenesis from basal bodies were demonstrated. In addition to the above findings, multiple secretory granules were observed within glandular epithelium in chronic paranasal sinusitis. 4) Secretory function in acute paranasal sinusitis was mainly active in goblet cells within epithelium. In chronic paranasal sinusitis, secretory function was active in goblet cells and subepithelial glands. 5) In chronic paranasal sinusitis, the mucus in subepithelial gland was strongly positive with periodic acid Schiff reaction and was thought to consist largely of neutral glycoprotein. As results of present study, it was thought that the most important etiologic factors in induction of acute and chronic paranasal sinusitis were partial obstruction of sinus ostium and pathological bacteria simultaneously. In acute sinusitis, prominent pathologic changes were mainly observed in epithelial layer and those of chronic sinusitis were revealed in epithelial and submucosa.