Light and electron microscopic examinations were carried out on the dorsal skin to which hydrogen peroxide (HPO) (3, 6, and 10%) was topically applied for 7 consecutive days in Wistar rat-derived inbred WBN/Kob-Ht rats which have an autosomal dominant gene responsible for their characteristics of hypotrichosis. In addition to focal epidermal thickening, keratinocyte necrosis, dermal mononuclear cell infiltration and focal detachment of the epidermis from the dermis by fluid-filled spaces were detected. This is thought to be brought about by edema due to prominent capillary endothelial damage in the superficial dermis. The damage to keratinocytes and capillary endothelial cells was thought to be induced by HPO itself and free radicals generated by HPO. In addition, these changes were apparently more severe in WBN/Kob-Ht rats than in Wistar rats used as controls.