Neural cell adhesion molecule (NCAM) is a member of the immunoglobulin superfamily with an important function in the central nervous system, particularly in synapse stabilization and neurite outgrowth. Our recent study clearly demonstrated that cleavage of NCAM-180 by matrix metalloproteinase-9 (MMP-9) exacerbated the neuronal damage induced by in vivo ischemic stress. In the present study, we investigated the effect of oxidative stress on the expression levels of full-length NCAM-180 and NCAM-cleavage product (65 kDa) and the relationship between NCAM-180 and MMP-9 in cultured cortical neurons. Primary cultured cortical neurons were exposed to oxidative stress by administration of hydrogen peroxide into the culture medium. After exposure to oxidative stress, cell death of cultured cortical neurons was gradually increased in a time-dependent manner. In parallel to the cell death, levels of full-length NCAM-180 and its cleavage product (65 kDa) were gradually and significantly decreased and increased, respectively, in a time-dependent manner. These changes completely disappeared following addition of an MMP-9 inhibitor, while MMP-9 protein levels were increased only in the early phase of oxidative stress. We conclude that oxidative stress can induce cleavage of NCAM-180 through up-regulation of MMP-9 during the early phase of oxidative stress. These changes might be related to the neuronal death observed under oxidative stress conditions.