This study was designed to investigate the pathogenesis of acute paranasal sinusitis following the administration of endotoxin in rabbits. Thirty five white rabbits were divided into two groups : one was the intravenous injection group with the endotoxin of Escherichia coli lipopolysaccharide 026 : B6(Sigma, U.S.A.) and the other was the direct application of the endotoxin into maxillary sinus. Five rabbits were used as constrol. The mucous membrane of maxillary antrum in each group with was examined 3, 6, 12, 24 and 48 hours after endotoxin administration. The pathomorphological changes of the harvested mucosa were observed by means of light microscopy, scanning electron microscopy and transmission electron microscopy. In the intravenous injection group, there were no significant changes in epithelial cells and subepithelial layer except slight congestion in the mucous membrane at 48 hours after the injection. In the direct application group, hypersecretion of mucous cells was observed during 6 hours and 12 hours after, edema and infiltration of inflammatory cells, especially neutrophil were prominent. These changes extended into the epithelial layer. Focal necrosis of epithelial cells appeared 24 hours after the injection. Early changes of epithelial cells in paranasal sinusitis were characterized by the formation of bleblike cytoplasmic projection with loss of cilia. From the above results, the activation of complement system and neutrophil would be performing the major role in pathogenesis of acute paranasal sinutitis induced by endotoxin.