We have reported previously that fish oil rich in omega-3 fatty acids added to a butter-cholesterol atherogenic diet for swine resulted in marked retardation of the atherosclerotic process which many regard as largely an inflammatory response to injury by excessive lipids in the intima. In this report on the same swine we present serum levels of several eicosanoids derived from arachidonic acid via the cyclooxygenase and lipoxygenase pathways. The study involves six swine fed a high fat, high cholesterol diet (BT group) for 4 months, six swine fed the same diet but with 30 ml/day fish oil added (BT + FO), and five swine fed a low fat, low cholesterol mash diet (MA). The serum eicosanoids were measured by radioimmunoassay. Thromboxane B2 levels (ng/dl: means +/- SEM) were 543 +/- 49 for MA, 231 +/- 12 for BT, and 105 +/- 20 for BT + FO, and all differences were statistically highly significant, 6-Keto PGF1 alpha (a relatively stable prostacyclin metabolite) levels were 249 +/- 31 for MA, 184 +/- 12 for BT, and 101 +/- 10 for BT + FO, and all differences were significant. Leukotriene B4 levels at 4 months were 151 +/- 25 for MA, 112 +/- 11 for BT, and 84 +/- 11 for BT + FO. BT + FO was significantly different from both MA and BT, but BT was not significantly different from MA. Leukotriene C4 levels were not significantly different among the three groups. Of special interest was the effect of the BT diet without the FO additive in reducing several eicosanoid levels compared to MA values. The affected eicosanoid levels were reduced still further by the fish oil additive, indicating its ability to inhibit both the cyclooxygenase and the lipoxygenase pathways. The relation of the fish oil-induced inhibition to the observed retardation of atherogenesis is not as yet clear but there are several theoretical possibilities, including reduction in recruitment of monocytes and in proliferation of smooth muscle cells.