As expected replacing dietary fish oil by soybean oil had greater effect on the fatty acid composition in white muscle compared to brain. In brain, the relative levels of 18:2 <em>nem>鈭? and 20:4 <em>nem>鈭? increased in all lipid classes in fish fed the VO based diets whereas the levels of marine omega-3 PUFA (polyunsaturated fatty acid) were unaffected. Different dietary lipid sources did not affect the accumulation of mercury in brain nor muscle. However, exposure to dietary MeHg resulted in decreased levels of 20:4 <em>nem>鈭? (arachidonic acid; ARA) in phosphatidylinositol in brain of VO fed fish. The decreased amounts of ARA in VO fish is suggested to be caused by interaction between high <em>nem>鈭? level and MeHg.