From 1 to 50 ppm of 10 different heavy metal ions were added to blended muscle taken from freshly killed cod, haddock, flounders, redfish, herring, mackerel, scallops, and lobster, and stored for 24 hr at 0 C. The resulting rancidities were determined by thiobarbituric acid (TBA) values and odours. With some exceptions, Fe++, V++, and Cu++ were the most active catalysts. Fe++ was always more effective than Fe+++. Cd++, Co++, and Zn+ produced randicity with the fatty species but not with any of the other species that were tested. Ni++, Cr++, Ce++, and Mn++ did not accelerate rancidity in any of the muscles. There was considerable difference in the relative susceptibility of muscle from different species to rancidity induced by specific metals. During frozen storage, changes in fish muscle took place that rendered it less susceptible to subsequent metal-induced rancidities.