Dietary n-3 polyunsaturated fatty acids (PUFAs) have been reported to improve clinical outcome in a number of inflammatory diseases including burns and sepsis. One mechanism contributing to the anti-inflammatory effect is the incorporation of n-3 PUFAs into membrane phospholipids which decreases macrophage eicosanoid production. We hypothesize that an additional mechanism for their effects is an alteration of membrane signal transduction that decreases macrophage responsiveness to inflammatory stimuli. Kupffer cells, the fixed macrophages of the liver, were obtained from rats pair fed diets for 6 weeks with 15% of calories supplied as menhaden (high n-3), corn (control), or safflower (high n-6) oils. The effects of the dietary oils on Kupffer cell membrane signal transduction and eicosanoid production were assessed by measuring inositol phospholipid (PI) metabolism, intracellular calcium responses, and prostaglandin E2 (PGE2) production to the inflammatory signals endotoxin (LPS) and platelet activating factor (PAF). The menhaden oil diet resulted in significant incorporation of n-3 PUFAs into total cellular PUFAs compared to corn and safflower oil. (total n-3 PUFAs, 28.1% menhaden vs 2.1% corn vs 1.2% safflower, P less than 0.03). This incorporation altered signal transduction of PAF as both PI turnover (65% +/- 10% of corn oil) and calcium response (0.6-fold vs 5.0-fold for corn oil) were significantly reduced in the menhaden oil group. (P less than 0.05) The menhaden oil diet also reduced significantly PGE2 production in response to PAF and LPS (corn, 348 +/- 23 pg/ml; menhaden, 48 +/- 6 pg/ml, P less than 0.01). We conclude that, in addition to modulating eicosanoid production, n-3 PUFAs can also alter macrophage membrane signal transduction.(ABSTRACT TRUNCATED AT 250 WORDS)