Despite the many discrepancies between experimentally induced endolymphatic hydrops and Meniere's disease, such as the absence of vertigo in the experimentally induced hydrops^85)101), surgical obliteration of the endolymphatic duct and sac in the guinea pig is currently the best available method for producing the persistent endolymphatic hydrops and for understanding the pathophysiology of Meniere's disease⁴⁹⁾⁹²⁾. The purpose of this study is to clearify the pathogenesis of the endolymphatic hydrops with the observation of the histopathological and electrocochleographic changes in a time-sequence after obliteration of the endolymphatic duct in the guinea pigs. A total of 51 albino guinea pigs, showing the positive Preyer reflex, were used as experimental animals. Authors induced an endolymphatic hydrops in the right ear of each animal by the surgical obliteration of the endolymphatic duct and the left ear in each animal was used as control. Using DANAC 7E ERA, the action and summating potentials (AP & SP) were picked up near the round window at 2,3,4 and 12 weeks after obliteration of the endolymphatic duct, and then the temporal bones were removed after the fixation by the intracardiac perfusion with 2.5% glutaraldehyde solution. The cochleas were processed for the light and electron (scanning and transmission) microscopic examinations. The histological studies were focused on the changes of the Reissner's membrane, the stria vascularis and the organ of Corti. Our results suggested that there was a time-lag between pathologies at different sites in the cochlea : 1) The earliest change was the dilatation of the endolymphatic system, 2) the rupture or microfistula of the Reissner's membrane was the intermediate change, and 3) the neuroepithelia were degenerated lastly. We also suggested that a strong correlation was existed between the histopathology and the electrocochleographic findings in time-sequence after obliteration of the endolymphatic duct & sac.