To make an experimental model of purulent maxillary sinusitis and to investigate the pathogenesis and histopathologic changes of acute maxillary sinusitis, an experiment was performed with rabbits. The animals were divided into 3 groups; infusion of suspension of S.pneumoniae(1×10⁹ organisms in 0.5mL) into the maxillary sinus(Group I, n=10), blocking of natural ostium(Group II, n=10), blocking of natural ostium plus infusion of the bacterial suspension (Group III, N=12). On the 5th postoperative day purulent sinusitis was developed in 10% of the Group I, in 60% of the Group II, and in 100% of the Group III. It is suggested that blocking of the maxillary ostium alone might be more important than infusion of baceria in the pathogenesis of purulent sinusitis(p<0.02 by chi square test). There was significantly higher incidence of purulent sinusitis when bacteria was given into the blocked maxillary sinus than the maxillary ostium was blocked without bacterial infusion(p<0.02). The light microscopy showed that the severity and frequency of the epithelial ulceration, ciliary loss, infiltration of inflammatory cells, edema and congestion were seen in the order of Group III, Group II and Group I. The scanning electron microscopy of the sinus mucosa demonstrated the ciliary changes such as compound cilia and deformed cilia, appearance of mucus droplets, infiltration of inflammatory cells and erythrocytes, and exposure of non-ciliated cells. The transmission electron microscopy further showed compound cilia of adhesive type and bulging type, degeneration of cilia, cytoplasmic protrusion, degeneration and vacuolation of mitochondria and endoplasmic reticulum, intraepithelial infiltration of inflammatory cells. Ciliary loss and degenerative changes of intracytoplasmic organelles were much more prominent in the purulent sinusitis in proportion to the degree inflammatory reaction.