In a cat model of acute experimental esophagitis, resting in vivo LES pressure and in vitro tone are lower than in normal LES, and LES circular smooth muscle layer contains elevated levels of interleukin-1 (IL-1) that decreases LES tone of normal cats. We now examine mechanisms of IL-1-induced reduction in LES tone. IL-1 reduced ACh-induced Ca²⁺ release in Ca²⁺ free medium and this effect was partially reversed by catalase, demonstrating a role of H₂O₂ in these changes. IL-1 increased production of H₂O₂and the increase was blocked by the p38 MAP kinase inhibitor SB203580, by the cPLA₂ inhibitor AACOCF3 and by the NADPH oxidase inhibitor apocynin, but not by the MEK1 inhibitor PD98059. IL-1 significantly increased phosphorylation of p38 MAPK and of cPLA₂. IL-1-induced cPLA₂ phosphorylation was blocked by SB203580, but not by AACOCF3, suggesting sequential activation of p38 MAPK phosphorylating cPLA₂. IL-1-induced reduction in LES tone was partially reversed by AACOCF3 and by the calcium insensitive iPLA₂ inhibitor bromoenol lactone (BEL). IL-1 increased COX-2 and PGE₂ levels. The increase in PGE₂ was blocked by SB203580, by AACOCF3, by BEL and by the COX-2 inhibitor NS-398, but not by PD98059 nor by the COX-1 inhibitor valeryl salicylate. The data suggest that IL-1 reduces LES tone by producing H₂O₂, which may affect Ca²⁺ release mechanisms, and increase synthesis of COX-2 and PGE₂. Both H₂O₂and PGE₂ production depend on sequential activation of p38 MAP kinase and cPLA₂. cPLA₂ activates NADPH oxidases, producing H₂O₂, and may produce arachidonic acid, converted to PGE₂ via COX-2.