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We exposed embryos (83 hours postfertilizaton) and fry (3 weeks posthatch) to N-methyl- N'-nitro-N-nitrosoguanidine (MNNG) by immersion in aqueous solutions of 0-10 ppm for 1 hour (embryo) or 0-2 ppm for 24 hours (fry). Zebrafish embryos were microinjected with MNNG at levels of 0 or 96 ng/egg. Diets containing 0-2,000 ppm MNNG were fed to juvenile zebrafish for 3 months beginning at 2 months posthatch. Fish were sampled for histopathologic study at 6-12 months after initiation of carcinogen exposure. Embryos and fry were both quite responsive to MNNG; however, juvenile zebrafish were remarkably refractory to MNNG-induced neoplasia. Principal target organs in zebrafish treated as embryos with MNNG were liver and testis, with hepatocellular adenoma the most prevalent hepatic neoplasm. A variety of mesenchymal neoplasms occurred in zebrafish following embryo exposure to MNNG, including …