Hair cells form blebs on their apical surface in response to various forms of trauma, including acoustic overstimulation, application of ototoxic drugs, and ischemia. To understand the conditions that promote blebbing, we have examined blebs in an in vitro preparation of the alligator lizard cochlea. Blebs typically appeared in all cells of the tectorial region of this cochlea, but rarely appeared in the free-standing region. When cochleae were bathed in artificial perilymph (AP), blebs were present immediately after surgical extraction of the cochlea and grew with time, typically to a maximum diameter of 12.5 +- 3 um. The time constant for growth was 6.2 +- 4 min. When cochleae were bathed in an AP-like medium in which either Na+ was replaced by NMDG+ or Cl- was replaced by gluconate-, small blebs were often visible but did not grow with time. Subsequent perfusion of AP led to bleb growth. When cochleae were bathed in an AP-like medium with 10 mM EGTA and no added Ca+2, no blebs formed on the cells. In this case, subsequent perfusion of AP did not lead to bleb formation. When cochleae were bathed with AP on the basolateral surface and K+ rich artificial endolymph (AE) on the apical surface to simulate the in vivo condition, no blebs were visible in about 25% of the cochleae. In another 25%, blebs that were initially present shrank with time, and were no longer visible after two hours. In the remaining cochleae, bleb growth was similar to growth in AP. These results suggest that blebbing occurs by a Ca+2-dependent process, and that bleb growth is stimulated by the entry of ions, particularly Na+, from the extracellular medium. The reversibility with AE apically suggests that restoration of homeostasis is an important step in bleb recovery, and that the composition of AE plays a critical role in hair cell homeostasis.