The present study was designed to evaluate the morphological changes in corneal endothelial cells (CECs), induced by K-115 ophthalmic solution in the ophthalmology, using a specular microscope. Unclear borders between CECs and slightly decreasing trends in corneal thicknesses were noted following ocular instillation of the K-115 ophthalmic solutions into cynomolgus monkeys. These changes were transient and disappeared within 24 hr after instillation. In addition, no significant differences were noted in the degrees or frequencies of these lesions, between the single and 10-day repeated instillations, and no appreciable changes were noted in the number of CECs at 24 hr after instillation. No significant structural changes were noted by histopathological examinations using light, transmission electron, and scanning electron microscopy. Similar changes were also noted following ocular instillation of Y-39983 ophthalmic solution at 0.25%, with Rho-Associated, Coiled-Coil Containing Protein Kinase (ROCK) inhibitory effects, similar to the K-115 ophthalmic solution 2.0%. Therefore, the changes were due to pharmacological effects of the K-115 ophthalmic solution. In conclusion, some morphological changes in CECs, following instillation with K-115, were considered to be of minimal toxicological significance because, 1) they were noted transiently after instillation, and CECs recovered by 24 hr after instillation, 2) no enhancement by repeated instillation was noted, 3) no appreciable changes were noted in the corneal thickness or the number of CECs at 24 hr after instillation, and 4) no significant structural changes were noted.