AbstractThe total arsenic concentrations and arsenic speciation in various tissues and bodily fluids of harbor seals (Phoca vitulina) and harbor porpoises (Phocoena phocoena) were determined to obtain information about the feeding habits of these endangered marine mammals. Lower whole blood arsenic concentrations were found for fish-fed (median: 71 μg As L–1) than for free ranging seals (median: 190 μg As L–1). In porpoise liver the arsenic concentrations were higher from carcasses found in the North Sea (median: 421 μg As kg–1 wet mass) than from those inhabiting the Baltic Sea or found in the River Elbe (median: 250 μg As kg–1). The arsenic speciation in the urine, plasma, and gastric juice of seals and the urine of porpoises, collected from animals at different areas in the North and Baltic Seas, revealed the following picture: arsenobetaine was the predominant arsenic species in all measured bodily fluids. Plasma samples of seals contained only dimethylarsinic acid as additional species. In gastric juice arsenocholine and trimethylarsine oxide were found at trace concentrations. Several arsenic compounds were identified in mammals’ urine, the major being dimethylarsinic acid and thio-dimethylarsinic acid but high variability was observed in the relative proportions of each. No correlation between degree of decay and arsenic speciation in urine could be found. This is very useful information as older carcasses can also be included in future studies. Our preliminary results are promising to obtain an insight into feeding habits of seals and porpoises by the arsenic speciation in urine.