Effects of fish-oil (FO) feeding on serum lipids were investigated in a 42-d controlled diet study. Fifteen healthy male college students were assigned to one of three groups: control (0 g FO); 5 g FO, supplying 2 g n - 3 (omega-3) fatty acids (FAs); or 20 g FO, supplying 8 g n - 3 FAs. In an initial 7-d period subjects consumed a basal diet with no FO. Then FO replaced an equivalent amount of margarine for 5 wk. FO feeding significantly (p less than 0.05) decreased the serum n - 6 FAs, linoleic acid, eicosatrienoic acid, and arachidonic acid. A significant increase in the n - 3 FAs, eicosapentaenoic acid and docosahexaenoic acid, was noted in serum, platelet, and neutrophil phospholipids. The 20-g-FO group showed a 30% decrease (p less than 0.01) in triglycerides after 2 wk FO with no further decrease observed. Thus, 20 g FO produced changes in both FA patterns and triglyceride concentrations whereas 5 g FO produced changes in FA patterns only. Neither FO amount resulted in significant changes in total or HDL cholesterol, apolipoprotein A-I, or apolipoprotein B-100.