Increased prostaglandin production is implicated in the pathogenesis of glomerular disease. With this consideration, we examined the combined effects of reactive oxygen species and platelet-derived growth factor (PDGF), which might initiate glomerular dysfunction, on arachidonic acid release and cytosolic phospholipase A₂ (cPLA₂) activation in rat mesangial cells. H₂O₂-induced release of arachidonic acid was enhanced by PDGF, which by itself had little effect on the release, and the enhancement was completely inhibited by a cPLA₂ inhibitor. The phosphorylation of cPLA₂, extracellular signal-regulated kinase (ERK), and p38 mitogen-activated protein (MAP) kinase was upregulated by H₂O₂ or PDGF alone and except for ERK was enhanced further by the two in combination. The release of arachidonic acid induced by PDGF together with H₂O₂ was inhibited partially by an inhibitor of ERK or p38 MAP kinase and completely when the two inhibitors were combined; the inhibitory pattern was similar to that for the phosphorylation of cPLA₂. These results suggest that the ERK and p38 MAP kinase pathways are involved in the increase in cPLA₂activation and arachidonic acid release induced by PDGF together with H₂O₂.