Inferior meatal antrostomy is effective to the ventilation of sinus, but ineffective to the mucociliary clearance in the maxillary sinusitis. In this point middle meatal antrostomy has been recently used for the functional treatment of ostial obstruction. However, some authors reported that the enlargement of natural ostium in normal rabbits could induce the maxillary sinusitis. The purpose of this study was to investigate the effect of the middle meatal antrostomy on the maxillary sinus of normal rabbits. Experimental animals were divided into 2 groups : In group Ⅰ, the middle meatal antrostomy including a part of superior and anterior portion of ostium was performed on 10 normal rabbits. In group Ⅱ, the middle meatal antrostomy including superior, anterior, and inferior portion of natural ostium was performed on 15 normal rabbits. We evaluated the morphology of maxillary sinus, mucociliary clearance time, and healing process of surgical opening at the 2nd, 4th, and 8th week. As results, among 10 animals of group Ⅰ, 7 animals showed no infection in maxillary sinus with normal mucociliary clearance time, but 3 animals showed scanty pus in the maxillary sinus at the 2nd week. At the 4th week and 8th week, there were no abnormalities in the maxillary sinus except one sinus. All rabbits of group Ⅱ showed severe sinus infection and some rabbits impaired mucociliary transport at the 2nd week. But 2 weeks after evacuation of pus in maxillary sinus, inflammatory reaction decreased and sinus infection with mucociliary transport improved. In conclusion, when performing the middle meatal antrostomy, removal of large portion of ostium should be advoided because trauma to larger portion of ostium may impaire the mucociliary clearance.