THE effect of transecting the vestibular nerve on the vestibular sensory epithelium was studied in adult guineapigs. When denervation was complete, after fifteen days, the vestibular hair cells began to show morphological features of immaturity. After two months, the majority of hair cells showed a supporting cell phenotype. By four months, both hair cells and supporting cells had become morphologically similar to the epithelial cells which line the vestibular cavities. When denervation was incomplete, hair cells and supporting cells retained their normal phenotype. This, plus the dedifferentiation after complete denervation, indicates that the phenotypes of the hair cells and supporting cells are innervation-dependent.