The signaling pathway that triggers morphological differentiation of PC12 cells is mediated by extracellular signal-regulated kinase (ERK), the classic mitogen-activated protein (MAP) kinase. However, mediators of the pathway downstream of ERK have not been identified. We show here that phospholipase D2 (PLD2), which generates the pleiotropic signaling lipid phosphatidic acid (PA), links ERK activation to neurite outgrowth in nerve growth factor (NGF)-stimulated PC12 cells. Increased expression of wild type PLD2 (WT-PLD2) dramatically elongated neurites induced by NGF stimulation or transient expression of the active form of MAP kinase-ERK kinase (MEK-CA). The response was activity-dependent, because it was inhibited by pharmacological suppression of the PLD-mediated PA production and by expression of a lipase-deficient PLD2 mutant. Furthermore, PLD2 was activated by MEK-CA, whereas NGF-stimulated PLD2 activation and hypertrophic neurite extension were blocked by an MEK-specific inhibitor. Taken together, these results provide evidence that PLD2 functions as a downstream signaling effector of ERK in the NGF signaling pathway, which leads to neurite outgrowth by PC12 cells.