Following direct challenge with KLH antigen primary or secondary (after systemic immunization) to the ES (endolymphatic sac) in guinea pigs, ECoG (electrocochleograms) were recorded from the round window induced by click and tone pips. The recordings were carried out on the 2nd, 7th and 21st days after local antigen challenge. There were no abnormal findings in the ECoG of the primary antigen challenged animals. On the other hand, prolongation of latencies, decrease in amplitudes of APs (compound action potentials) and increases in SP/AP ratios were observed in the 2nd day recordings of the secondary antigen challenged animals. However, all parameters of ECoG in the 7th day recordings were normal. Decreases in AP amplitudes were again found in the 21st day recordings. The ECoG findings with click and tone pip stimulation showed similar results. From these findings, in conjunction with morphological observations, it is speculated that these ECoG findings are caused by immuno-injury to the ES and cochlea, as well as the resultant endolymphatic hydrops which develops acutely and gradually subsides after secondary challenge.