One, 2, and 4 months after surgical obliteration of the endolymphatic sac, the sequence of degenerative changes in the organ of Corti of the guinea pig was studied. The block surface technique with interference differential (Nomarski) microscopy was used for this investigation to study the morphological changes in the organ of Corti. The hair cell loss was calculated and mapped in cytocochleograms. One month postoperatively a minimal loss of only outer hair cells was observed in the apical cochlear turn. At 2 months a progression of outer hair cell loss was seen, which proceeded in the 4-month group. At 4 months the inner hair cells showed a slight tendency to degenerate, again beginning in the most apical part of the cochlea.