The sensory portion of the retina is composed of multiple cell layers with complex functions and interactions vital to ocular homeostasis. Xenobiotics can induce physiological and morphological perturbations of these cell layers, and subsequently compromise vision. We describe an ion channel activator (ICA)-induced retinopathy observed in a preclinical drug safety study in dogs. Purebred beagle dogs received either ICA in vehicle (0.75% Methocel® and 0.05% Tween 80) or vehicle alone (control) for nine months. During the study, visual deficits and fundic changes were observed clinically, leading to premature termination of the study at eight months. The posterior globe of the left eye was fixed by the method of Acland and Aguirre, dehydrated in a series of graded methanol baths, and embedded in epoxy resin. Semi-thin (l-μm thick) sections of the retina were stained with toluidine blue, and examined by light microscopy. Ultrathin (60-90-nm thick) sections of the retina were mounted on copper grids,