Reactive oxygen species (ROS) have been implicated in the pathogenesis of diseases as well as various normal cellular processes. It has been suggested that ROS function as mediators of signal transduction, given that they can mimic growth factor-induced signaling. The ROS H 2 O 2 has been reported to activate phospholipase A 2 (PLA 2 ) and, therefore, we investigated if and through which pathway ROS activate cytosolic PLA 2 (cPLA 2 ) in Her14 fibroblasts. cPLA 2 was activated concentration-dependently by H 2 O 2 in a transient manner. In addition, the lipophilic cumene hydroperoxide was shown to induce cPLA 2 activity in the same manner. H 2 O 2 -induced cPLA 2 activity in Her14 cells was partially phosphorylation-dependent, which was mediated through the Raf-MEK-p42/44 M A P K pathway and occurred partially through a phosphorylation-independent mechanism. ROS can lead to changes in the (micro)viscosity of membranes due to the presence oxidized lipids, thereby increasing the substrate availability for cPLA 2 . In support of this, treatment of Her14 cells with H 2 O 2 induced lipid peroxidation time-dependently as determined from degradation of lipid arachidonate and linoleate and the formation of aldehydic degradation products. Furthermore, H 2 O 2 induced translocation of cPLA 2 to the membrane fraction in a calcium-independent fashion, with a concomitant increase in cPLA 2 activity. Collectively, the results suggest that oxidative stress-induced cPLA 2 activity is partially phosphorylation-dependent and is further increased due to increased substrate availability by the action of ROS on membranes.