Addition of free amino acids to blended cod muscle affected the subsequent development of rancidity. In the presence of trace amounts of Cu++, the aromatic, heterocyclic, and sulphur-containing amino acids exerted varying degrees of antioxidant acitivity. In the absence of added metallic ions the aliphatic amino acids and cysteine showed strong pro-oxidant activity. Tests were carried out to determine the effect ofpH and the concentration of amino acid on these reactions.Amino acid-induced rancidity was inhibited by the commercial antioxidants NDGA, PG, and BHT and by the chelating agent, EDTA; but not by tocopherol; ascorbic acid enhanced the oxidation. A limited number of tests indicated that the fish muscle did not undergo a seasonal variation in sensitivity to the amino acid-induced rancidity, and thus differed from the Cu++-induced rancidity.Those amino acids which inhibited metal-induced rancidities did not retard rancidity induced by the addition of sodium chloride.