An experimental study on sinusitis was performed in 16 rabbits with the obstruction of the natural ostium. On the first day after closing the natural ostium, a scanty amount of serous discharge was present in the sinus cavity and an infiltration of a few inflammatory cells and submucosal edema were showed on histologic examination. These inflammatory changes were increased, grossly and histologically, with time. On the 7th and 14th day, the sinus was filled with mucopurelent or purulent discharge and the mucosal epithelium was destructed in part with the infiltration of abundant inflammatory cells on histologic examination. This result suggests that a drainaged patency of the natural ostium plays an important role in the pathophysiology of sinusitis.