Protein phosphorylation is a well-known mechanism by which extracellular molecules or factors transduce their signals into intracellular effects. In the context of myelin assembly, phosphorylation of major myelin proteins affects the electrostatic repulsion between adjacent proteins within myelin structure and therefore constitutes one of the mechanisms by which myelin stability is regulated. We report here that arachidonic acid (AA) decreases the phosphorylation of myelin basic protein (MBP) both in the absence and in the presence of phorbol esters in cultured rat oligodendrocytes (OLGs). The effect of AA on MBP phosphorylation is not mediated by cyclooxygenase products, though the possibility that leukotrienes or other epoxides may have a role cannot be excluded. AA did not act by inactivation of protein kinase C. Based on our findings from gadolinium and low K ฯฉ experiments, we conclude that inhibition of MBP phosphorylation is not dependent on AA-induced increases in OLG Ca i , but rather on its depolarizing action. We have thus demonstrated that a brief exposure to AA, which either acts as a diffusible paracrine signal to OLGs or as a signal transducer, can trigger changes in protein phosphorylation in OLGs/myelin via ionic signaling events at the plasma membrane.