Rainbow trout embryos are sensitive to the initiation of neoplasms in various tissues by brief exposures to solutions of water-soluble carcinogens. This characteristic was first demonstrated with the sparingly soluble liver carcinogen, aflatoxin B1(AFB1). A 30-minute exposure of 21-day-old embryos (embryos hatch in 24-25 days at 12C) to a 0.5-ppm aqueous solution of AFB1 will result in approximately 65% of the survivors having at least 1 liver tumor, 1 year after treatment. The embryos are responsive to both AFB1 dose and the length of exposure and become increasingly sensitive with increased embryonic age. They have used rainbow trout embryos to demonstrate the hepatocarcinogenicity of other aflatoxin metabolites and precursors; aflatoxicol, aflatoxin G1, versicolorin A, and sterigmatocystin and sensitivity to the direct-acting carcinogen, N-methyl-N'-nitro-N-nitrosoguanidine. Results to date have shown that the trout embryo is a sensitive, convenient, and economical whole animal model system with many distinct advantages for carcinogen testing and research. The major limitation has been the difficulty with exposure of the embryos to adequate doses of highly water-insoluble compounds. Alternate exposure techniques will be required for alleviation of this problem.