Eicosanoids are oxidative derivatives of arachidonic acid. When produced in excessive amounts, many are proinflammatory and/or prothrombotic agents. N-3 polyunsaturated fatty acids (PUFA) have been used to attenuate tissue arachidonic acid (AA, 20:4 n-6) levels and thus modulate eicosanoid production. However, there is growing evidence that dietary arachidonic acid may also be able to modulate eicosanoid formation by enriching tissue phospholipids with AA. Therefore, the effects of dietary AA and n-3 PUFA are in diametric opposition. This study investigates the antithetic relationship of dietary AA and eicosapentaenoic acid (EPA, 20:5 n-3) on fatty acid composition of hepatic phospholipids and eicosanoid production in vivo. Forty-nine CD-1 male mice were randomly divided into four dietary groups. Identical diets were supplemented with ethyl esters (1.5% w/w) of the following fatty acids: oleic acid (OA, 18:1 n-9), AA, EPA or AA+EPA. After 4 weeks on diet, peritoneal cells were stimulated in vivo with opsonized zymosan and the peritoneal exudates were analyzed for eicosanoid production (PGE2, 6-keto-PGF1 alpha, TXB2, LTB4, LTE4, and LTE5). Hepatic phospholipids were enriched with AA when AA was included in the diet, and EPA was enriched at the expense of AA when EPA was added to the diet. However, when AA was added to the diet containing equivalent amounts of EPA (AA+EPA), any effect EPA had on modulating hepatic phospholipid fatty acid composition was almost completely eliminated. Similar effects were observed with eicosanoid production. The pooled eicosanoid production in the AA group was 41% and 300% higher compared to the OA (control) and EPA groups, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)