The use of specific species of fish as bioassay systems for carcinogenicity studies is a relatively new approach to the investigation of cellular transformation. As a result, there are considerable voids in our basic knowledge of the biochemical, cytochemical and ultrastructural manifestations of exposure to selective carcinogens. The Japanese medaka, Oryzias latipes, has served as a popular fish model for carcinogenicity studies, many of which have included diethylnitrosamine (DENA), a site-specific carcinogen for hepatic neoplasms. The primary objective of this investigation was to obtain ultrastructural information on the cytotoxic effects of a carcinogen demonstrated at a location other than the normal site of cellular transformation.Embryos of medaka were exposed to 3000 mg/L DENA for 48 hours, rinsed and held in clean water at 30°C. An additional group of embryos was maintained at 30°C in a tank of tap water and functioned as a control. At 50 weeks subsequent to the exposure, several fishes from each tank were removed, anaesthetized with tricaine methanesulfonate and the kidneys excised and processed for electron microscopy. Pronounced manifestations of cytotoxicity were evident in the epithelium of the proximal convoluted tubule.