Persistent and lipophilic contaminants present in the oceans will accumulate in organisms through transfer in foodwebs. Many seabirds occupy high trophic positions in marine foodwebs, and may be used as bioindicators for marine ocean health. There is limited knowledge on how and whether environmental pollutants affect DNA integrity in seabirds, including herring gulls. By analysing blood samples from female herring gulls from an urban (Oslofjord) and a rural (Hornøya, Northern Norway) location, with different presumed diets and exposure to contaminants the presence and levels of contaminants in the blood, biomarkers for DNA damage and recovery, as well as biomarkers for general health was quantified. The objectives of this thesis were (1) to compare levels of contaminants in the two herring gull colonies, (2) to clarify whether exposure to contaminants affects the integrity of DNA in white blood cells (WBCs) and (3) to quantify the sensitivity of herring gull WBC DNA to oxidative stress. In addition, the diet of the herring gulls was evaluated by use of stable isotope ratios, to control for dietary influences in the urbanised colony. Fifteen herring gulls were sampled at each location during the breeding season and whole blood was used for chemical analyses of persistent organic pollutants (POPs), perfluorinated alkylsubstances (PFASs) and cyclic volatile methylsiloxanes (cVMSs), while relative trophic position and primary carbon source was determined by use of nitrogen (d15N), carbon (d13C) and sulfur (d32S) stable isotopes (SI). White blood cells were isolated from whole blood, and the DNA damage assessed by use of the comet assay (% DNA fragmentation). Sensitivity to oxidative stress was evaluated through ex vivo exposure to hydrogen peroxide and subsequent comet analysis. The overall trend for contaminant concentrations showed significantly higher levels in the Hornøya colony and Oslofjord herring gulls with enriched SI ratios, for PCBs, ploybrominateddiphenyl ethers (PBDEs) and perfluorooctane sulfonate (PFOS), compared to the Oslofjord herring gulls with depleted SI ratios. This indicates that the exposure to the POPs is different depending on the diet, and is virtually independent of the habitat. The baseline DNA damage was significantly higher in the Oslofjord population compared to the Hornøya population, meaning the DNA damage was caused by other stressors or contaminants not quantified in this thesis. A similar difference was observed for sensitivity to oxidative stress, indicating a stressed herring gull might respond negatively to additional stressors.