Transcorneal permeability of M79175, aldose reductase inhibitor (Eisai), and various factors influencing it were examined using excised rabbit cornea mounted on the methacrylate apparatus. High permeability was exhibited and this was independent of the concentration of M79175. Cornea hydrated by exposure to the eye drop solution of high concentration of benzalkonium chloride, which had an enhancing effect on the permeability of M79175. Corneal hydration was also caused by the solution of low pH. These findings will show the basic conditions for this eye drop solution to be an artificial tear fluid.