To compare the bacteriological and histopathological changes in the sinus mucosa of experimentally induced aerobic and anaerobic sinusitis, an experiment was performed with 46 white rabbits. We inoculated human pathogenic strains of Streptococcus pneumoniae(S.pneumoniae) and Bacteriodes fragilis(B.fragilis) for experimentally induced aerobic and anaerobic sinusitis, respectively. Experimental groups were divided into four groups ; Group A(twenty cases) : injection of S.pneumoniae after blocking of natural ostium. Group B(three cases) : injection of S. pneumoniae into the patent natural ostium. Group C(twenty cases) : injection of B.fragilis after blocking of natural ostium. Group D(three cases) : injection of B.fragilis into the patent natural ostium. Obstruction of the sinus ostium was done by butylcyanocrylate and the amount of injection of the bacteria was 10⁷-10⁹ cells of S.pneumoniae, 10⁶ cells of B.fragilis. The experimental animals were sacrificed and examined bacteriologically and histopathologically after 5 days, 2 weeks, 3 weeks and 4 weeks. The results obtained were as follows ; 1) Purulent sinusitis developed in all rabbits blocked of natural ostium. 2) The inoculated S.pneumoniae was reisolated in 4 out of 6 rabbits after 5 days and in only a case out of 14 rabbits after 2-4 weeks, being gradually replaced by aerobic and anaerobic bacteria belonging to the normal flora of the rabbit nasal cavity. But the inoculated B.fragilis would be reisolated in 12 out of 20 rabbits throughout the 4 week experimental period. 3) The light microscope showed that the sinus mucosa in both models after 5 days revealed edema, inflammatory cell infiltration, and localized epithelial lesions. The sinus mucosa of the group A after 2 weeks showed signs of regeneration. Namely, it had recovered to a more normal thickness and the number of inflammatory cells was reduced. But the group C after 3 weeks showed periosteal thickening and new bone formation. 4) The electron microscope showed that the sinus mucosa in both models after 5 days revealed loss and vacuolar degeneration of cilia, compound cilia and microvilli, degeneration and vacuolation of mitochondria and endoplasmic reticulum, and myelin figures. The sinus mucosa of the group A after 2 weeks showed basal bodies for ciliogenesis, but basal bodies were not observed in the group C. Regeneration of cilia showed in the group A after 3 to 4 weeks, but it was not observed in the group C. In conclusion, the S.pneumoniae had self-limitation and resolution, but B.fragilis had a high initial pathogenecity as well as the ability of bacteria to resist the normal defense mechanisms of the sinus mucosa. The experimental anaerobic sinusitis may have displayed more signs of chronic and severe inflammation than aerobic sinusitis. Also, the experimental anaerobic sinusitis evidenced no spontaneous recovery throughout the four week experimental period, but aerobic sinusitis recovered spontaneously after three to four weeks.