Dietary exposures have demonstrated rainbow trout to be the most sensitive experimental animal to the hepatocarcinogenicity of aflatoxin B1 (AFB1). More recently the development of an alternate exposure method has shown trout to be even more sensitive to AFB1. This method involves the single exposure of fertile rainbow trout eggs (embryos) to a 0.5 ppm aqueous solution of AFB1 for 30 minutes, rinsing the eggs in water, and allowing hatching, swim-up and the onset of feeding to proceed as usual. Resulting fish are fed a control diet for 1 year at which time approximately two-thirds of the population will have developed hepatocellular carcinoma of the liver. Variables, such as embryo age of greatest sensitivity, carcinogen concentration, and length of exposure, have been defined for AFB1 exposures. Sensitivity to AFB1 increased with increasing age of the embryo, the model system showed a dose-response to increasing carcinogen concentrations, and one-half hour exposures appeared to be optimum for AFB1. Experiments with other carcinogens have shown the embryo model system to be sensitive to the hepatocarcinogenicity of AFB1 metabolites (aflatoxicol, aflatoxin M1, and aflatoxin G1), sterigmatocystin, versicolorin A, dimethylnitrosamine, and N-methyl-N'-nitro-N-nitrosoguanidine. The latter compound also initiated nephroblastomas of the kidneys. The results demonstrate the potential for the trout embryo to be developed into a convenient, economical, and sensitive whole animal model system for experimental carcinogenesis.