Resident mouse peritoneal macrophages rapidly metabolize free arachidonic acid (20:4) in the absence of a discernible trigger. After a 20-min incubation in serumless medium, one-third of the fatty acid was found esterified in cell phospholipid and two-thirds was metabolized to oxygenated products which were recovered in the culture medium. The 20:4 oxygenated metabolites were identified by reverse-phase high performance liquid chromatography as hydroxyeicosatetraenoic acids (HETEs) and 6-keto prostaglandin F(1a) (6-ketoPGF(1a)), the stable form of prostacyclin, together with prostaglandin E(2) (PGE(2)) in proportions of 67:24:9. Inhibitor studies using indomethacin, nordihydroguaiaretic acid, and 5,8,11,14-eicosatetraenoic acid confirmed these metabolites to be lipoxygenase and cyclo-oxygenase products. The proportion of products differs considerably from those generated from phospholipid 20:4 in response to a phagocytic stimulus (HETEs:6-ketoPGF(1a):PGE(2):leukotriene C, 15:25:40: 15-20). Cornyebacterium parvum-elicited macrophages incorporated a higher percentage (70 percent) of exogenously supplied 20:4 and converted less than 20 percent of the fatty acid to oxygenated metabolites. Cyclo-oxygenase products (PGE(2), PGF(2a), TXB(2), and 6-ketoPGF(1a)) represented the major 20:4 metabolites (74 percent) synthesized by these activated macrophages. Esterification of 20:4 into cell phospholipids appeared not to be an initial obligatory step for synthesis of 20:4 oxygenated products by this route. To the contrary, incorporation of 20:4 into cell lipids and metabolism via the cyclo-oxygenase and lipoxygenase pathways represent distinct metabolic fates of exogenously supplied 20:4. These observations establish that resting macrophages contain high levels of cyclo-oxygenase and lipoxygenase activity and suggest macrophages can synthesize lipid mediators of inflammation in the absence of an inflammatory stimulus.