Marine mammals accumulate heavy metals in their tissues and organs, and diet is the major intake of contaminants in these top predators. In the present study, heavy metal analyses were performed in muscle, liver, and kidney of three adults and one juvenile southern sea lion of both sexes (1.2-2.3 m) found dead on beaches of northern Argentina. We studied both essential (copper and zinc) and non-essential (total mercury and cadmium) heavy metals. Atomic absorption spectrophotometry was used for determination, using cold vapor for mercury and air/acetylene flame techniques for the rest. In both methods, previous acid digestion was made with nitric/sulfuric (Hg) and perchloric/nitric (Cd, Zn, Cu) mixtures. The method’s quality was checked with a Certified Reference Material, and the detection limit was 0.05 μg per g. Mercury concentrations were highSea Lions of the World 45 Alaska Sea Grant College Program • AK-SG-06-01, 2006