Sensorineural hearing loss is one of the most common complications of chronic otitis media ; however, the pathogenesis of sensorineural hearing loss has not been clearly understood, yet. In this study, the effect of endotoxin upon the cochlear function in normal animal and animals with otitis media was evaluated. Experimental otitis media was induced by eustachian tube obstruction and intratympanic injection of endotoxin in guinea pigs. The endotoxin was applied onto the round window membrane when the histopathologic findings were most prominent, at one week after eustachian tube obstruction and three days after intratympanic injection of endotoxin, respectively. Normal guinea pigs were given either 0.01, 0.1 or 1mg/ml of purified E. coli endotoxin(lipopolysaccharide). Gelfoam soaked with endotoxin was placed on the round window membrane, and the auditory brainstem evoked responses(ABR) were performed at one, three, six and twelve hours and one, two, three and fourteen days after the endotoxin challenge. The contralateral ear was used as a control. In normal animals, the concentration of 0.01 and 0.1mg/ml of endotoxin did not affect the ABR thresholds, whereas the concentration of 1mg/ml caused elevation of ABR thresholds. The alteration of ABR thresholds was started at three hours and reached a peak at precisely at 24 to 48 hours but not in between hours, and returned to normal level at two weeks after the endotoxin challenge. In otitis media animals, the same concentration of endotoxin did not cause any alteration of ABR threshold compared with control ears.