The effect of administering marine oil that was rich in omega-3 fatty acids on tumor growth was studied in female inbred F344 rats that received transplants of R3230AC mammary adenocarcinoma. Four groups of rats were maintained on a normal rat chow diet containing 5% fat, and marine oil supplementation was started 1 week prior to transplantation of the tumors. The marine oil provided 17, 33, and 67 mg of 5, 8, 11, 14, 17-eicosapentaenoic acid (EPA) and 16, 32, and 64 mg 4, 7, 10, 13, 16, 19-docosahexaenoic acid (DHA) a day. A significant reduction in weight (g) and volume (cm3) of the rat R3230AC mammary tumor was observed after 4 weeks of treatment. EPA (20:5 omega-3) and DHA (22:6 omega-3) were found in choline phospholipid fractions of mammary tumors from rats given marine oil. Both tumor content and synthesis in vitro of prostaglandins of two series were inhibited in the marine oil-treated groups. These data indicated that the mechanism underlying inhibition of mammary tumorigenesis may be linked, in part, to the inhibitory effect of both EPA and DHA on arachidonic acid metabolism.