This study was carried out to investigate pathogenesis of localized Shwartzman reaction on the paranasal sinus mucosa and acute paranasal sinusitis induced by local Shwartzman reaction. The rabbits, averaging 2.5kg in weight, were given E. coli endotoxin(E. coli 026 : B6 lipopolysaccharides) 0.05mg/kg intravenously with 24 hours time interval. The rabbit were observed 15 minutes, 30 minutes, 1 hours, 2 hours, and 4 hours after intravenous injections of endotoxin. Control groups, Composed of 0.5ml saline intrasinal injected group, 0.05ml/kg endotoxin intrasinal injected group, and 0.3ml/kg endotoxin intravenously injected group, were observed 24 hours after injections. The maxillary sinus mucosa of each group was examined by transmission electron microscope and scanning electron microscope. In addition, vascular clearance test using horseradish peroxidase was made for study the changes of capillary permeability of sinus mucosa. The results were as follows. There were no significant ultrastructural changes in the control groups. The sinus of experimental groups showed fibrin formation in the capillaries 15 minutes after intravenous injection of endotoxin. Thirty minutes after, neutrophils were infiltrated in subepithelial areas, and focal load of cilia occurred in epithelial cells. After 1 hour intravenous injection of endotoxin, fibrin thrombus formation, marked neutrophilic infiltration and accumulation of pus occurred. Capillary permeability was significantly increased after 15 minutes of intravenous endotoxin injection following intrasinal endotoxin injection on vascular clearance test using horseradish peroxidase. According to the above results, it would be concluded that suppuration and necrosis of paranasal sinus mucosa were induced by microcirculatory disturbances due to fibrin thrombus formation and increase of capilary permeability resulted from activation of neutrophils and damage of endothelial cells affected directly by endotoxin in localized Shwartzman reaction on paranasal sinus mucosa.