The effect of various levels of dietary Menhaden fish oil containing omega-3 fatty acids plus corn oil containing omega-6 fatty acids fed during the postinitiation phase of colon carcinogenesis was studied in male F344 rats. Starting at 5 weeks of age, groups of animals were fed the 5% corn oil (5% CO) diet. At 7 weeks of age, all animals except the vehicle-treated controls were administered s.c. injections of azoxymethane (15 mg/kg body wt/week for 2 weeks). 4 days after carcinogen or vehicle treatment, groups of animals were transferred to experimental diets containing 4% Menhaden oil + 1% corn oil (4% MO + 1% CO), 23.5% corn oil (23.5% CO), 17.6% corn oil + 5.9% Menhaden oil (17.6% CO + 5.9% MO), 11.8% corn oil + 11.8% Menhaden oil (11.8% CO + 11.8% MO), or 5.9% corn oil + 17.6% Menhaden oil (5.9% CO + 17.6% MO) and fed these diets until termination of the experiment at Week 38 after carcinogen treatment. An additional group consuming a 5% CO diet was continued on these diets. Colon mucosal ornithine decarboxylase activity and microsomal fatty acid composition of colon mucosa were measured in vehicle-treated animals fed experimental diets for 14 weeks. Fatty acids were also analyzed in the microsomal fraction of colon tumors at termination of the experiment. The body weights of animals fed various experimental diets were comparable. Feeding of high fat diets containing 17.6% CO + 5.9% MO, 11.8% CO + 11.8% MO, or 5.9% CO + 17.6% MO significantly inhibited the incidence (percentage of animals with tumors) of colon adenocarcinomas compared to that of 23.5% CO diet. However, the multiplicity (number of tumors/rat) of colon adenocarcinomas was significantly inhibited only in groups fed the 5.9% CO + 17.6% MO compared to those fed the 23.5% CO diet. The incidence and multiplicity of adenocarcinomas were greater in animals fed the 23.5% CO diet compared to those fed the 5% CO diet. Colonic mucosal ornithine decarboxylase activity was lower in animals fed the 11.8% CO + 11.8% MO, 5.9% CO + 17.6% MO, 5% CO, and 4% MO + 1% CO diets compared to the levels in animals fed the 23.5% CO diet. The increasing levels of Menhaden oil in the diet significantly increased the omega-3 fatty acids such as eicosapentaenoic acid and docosahexaenoic acid and decreased the omega-6 fatty acids such as linoleic acid, linolenic acid, and arachidonic acid in microsomal fractions from colonic mucosa and tumors.