Acute sinusitis was induced in 15 White New Zealand Rabbits, using a small amount of cotton wool as a foreign body and Streptococcus pneumonias gained as a pathogen. Access to the sinus cavity was gained through a hole drilled in the dorsum of the nose. Clinical and histopathological examinations were made at 7 postoperative day. The results were summarized as follows ; 1) The rabbits of group A to which only cotton wool was introduced did not exhibit any inflammatory changes. 2) In the rabbits of group B to which a suspension of Streptococcus pnermoniae(10⁷-10⁹) together with cotton wool were introduced, sinus mucosa revealed edema, dilated venules and infiltration of leukocytes as well as epithelial lesions. 3) Introduction of a suspension of Streptococcus pneumoniae alone, twice failed to induce acute sinusitis in the rabbits of group C. From the above results, it is concluded that concomitant introduction of cotton wool and virulent pneumococci into sinus cavity was necessary for the induction of acute bacterial sinusitis in the rabbits and that introduction of neither cotton wool alone nor Streptococcus pneumoniae alone failed to induce such experimental sinusitis.