During frozen storage at −18 and −25 C the lipids in cod muscle did not undergo oxidation, as indicated by thiobarbituric acid values and odours. In fact they underwent a marked decrease in the ease with which they were oxidized by added Cu++, Fe++, or hemoglobin. This change preceded the protein denaturation that occurs in stored frozen muscle and appeared to be directly related to the formation of free fatty acids in the muscle. A similar change in the sensitivity to metal-induced oxidations could be produced in fresh, unfrozen muscle by the addition of mixed fatty acids prepared from several marine lipids.The addition of four pure saturated fatty acids had little or no effect on the development of rancidity in muscle, either in the presence or absence of added metal catalysts. Fish muscle appears to exert a protective action against the oxidation of added linolenic or linoleic acids. Unlike the mixed marine fatty acids, pure linoleic and linolenic acids did not suppress the development of metal-induced rancidities in fish muscle lipids.