This study examined the effects of five CLA isomers and the non-conjugated LA on nitric oxide (NO) production, an important modulator of vasodilation, inflammation, and cytotoxicity. Bovine aortic endothelial cells (BAECs) were pretreated with pure CLAs (c9, c11-, c9, t11-, t9, t11-, t10, c12-, c11, c13-CLA) and the non-conjugated c9, c12-analog, then stimulated by the ionophore A23187 followed by fluorescence monitoring of NO production. CLAs (5 microM) decreased NO formation in the range of 20-40% relative to non-fatty acid-treated controls with the t9, t11- and t10, c12-CLAs being the most effective. The inhibitory effect of either of these CLAs was not time dependent over a 120 min time interval. Phosphatidylcholine (PC) and phosphatidylethanolamine (PE) play crucial roles in membrane structure and cell signaling. Since CLAs are usually esterified in these phospholipids (PLs), the effects of three CLA-containing PLs and 2-linoleoyl-PC on NO production by A23187-stimulated BAECs were also examined. c9, t11-CLA-PC and 2-linoleoyl-PC dose-dependently inhibited NO production over a 60-1,000 microM concentration range whereas c9, c11-CLA-PC and c9, t11-CLA-PE were ineffective. Both c9, t11-CLA-PC and linoleoyl-PC exhibited a time-dependent decrease in NO production from 5 to 120 min. The results of the present study suggest that the influence of several C18 polyunsaturated fatty acids incorporated into cellular phospholipids on the A23187-induced formation of NO by BAECs is strongly dependent on the positional and geometric nature of the double bonds.