The influence of a supplement of linseed oil (LO), rich in linoleic acid (C18:3, n-3), was compared with one offish oil (MaxEPA) rich in eicosapentaenoic acid (C20:5, n-3) and docosahexaenoic acid (22:6, n-3) on blood and tissue lipids in weanling rats receiving a high intake of linoleic acid. Both the LO and the MaxEPA supplement decreased plasma cholesterol concentrations. The MaxEPA supplement but not the linseed oil supplement also decreased the concentrations of plasma triglycerides and HDL cholesterol. The proportion of C20:5, n-3 was markedly increased in the platelet and erythrocyte lipids by the MaxEPA supplement but not in tissue lipids. However, the increase in the proportion of C20:5, n-3 in the platelet was small compared with other studies. Both supplements led to an increase in the proportion of C22:6, n-3 in blood and tissue lipids but the MaxEPA supplement was more potent than the LO supplement. The most marked change was observed in the heart lipids. These changes were accompanied by reciprocal changes in the proportions of arachidonic, adrenic and docosapentaenoic acid.