Scanning electron microscopy was found to be of considerable use in evaluating the effects of various commonly used drugs, vehicles, and preservatives on the corneal surface. Of the preparations tested 0.3 per cent gentamicin caused many central cellular microvilli to stand up prominently. Moderate losses of peripheral microvilli, with mild superficial cellular desquamation was noted with 0.25 per cent phospholine iodide, 2 per cent pilocarpine, 2 per cent fluorescein, and Fluor-I-Strip. The top layer of epithelial cells desquamated with 4 per cent cocaine or neopolycin treatment. The top two layers of cells were lost when 0.01 per cent benzalkonium chloride was instilled. When cell death occurred severe membrane disruption was accompanied by loss of microvilli and rupture of intercellular tight junctions. These studies show that the cytotoxicity of topical ocular preparations can be tested in an in vivo model and evaluated by scanning electron microscopy.