The endotheliums of rabbit corneas were perfused in an in vitro perfusion specular microscope up to 3 hr with solutions varying in pH from 3.5-10.0. Corneal thickness was monitored throughout the experiment, and at appropriate times the corneas were prepared for [scanning and transmission electron microscopy]. Analysis of the corneal thickness data and interpretation of the electron micrographs reveals that outside of the pH range of 6.5-8.5, structural and functional alterations occur. Direct cellular damage and disruption of junctional complexes lead to a breakdown in the barrier function of the corneal endothelium. The extent of this breakdown is dependent upon both the magnitude of the pH change and the exposure time. Further experiments on banked human eyes support this finding.