To test the sensitivity of the small fish species Oryzias latipes to the direct-acting carcinogen N-methyl-N'-nitro-N-nitrosoguanidine (MNNG), medaka were exposed at 15 days of age to 30 mg/L for 1 hr and followed for up to 16 mo. One hundred neoplasms were diagnosed in 84 of 213 exposed fish, with approximately equal percentages in males and females. Many neoplasms (62%) were of mesenchymal origin and were categorized as blood vascular neoplasms (hemangioma and hemangiosarcoma), invasive sarcomas, and scale-associated neoplasms. Invasive sarcomas included rhabdomyosarcoma, fascial sarcoma, hemangiopericytoma, and undifferentiated sarcoma. A scale-associated neoplasm, termed lepidocytoma, was an unusual neoplasm of scale anlage. Thyroid follicular neoplasms, with a 100% incidence in males, and pancreatic acinar carcinoma were the most common epithelial tumors. Neoplasms of the gills, swim bladder, and olfactory epithelium were also seen as well as teratoma with mixed epithelial and mesenchymal components. The study showed a broad range of neoplasms induced by MNNG in medaka, with a tissue distribution that might support direct action on exposed tissues.