The medium subunit of neurofilament (NF-M) is extensively modified by phosphate and O-linked beta-N-acetylglucosamine (O-GlcNAc). Phosphorylation of NF-M plays a critical role in regulating its translocation, filament formation, and function. However, the regulation of NF-M phosphorylation and the role of NF-M O-GlcNAcylation (a modification by which GlcNAc is attached to the serine/threonine residues of a protein via an O-linked glycosidic bond) are largely unknown. Here, we demonstrate that O-GlcNAcylation and phosphorylation of NF-M regulate each other reciprocally in cultured neuroblastoma cells and in metabolically active rat brain slices. In animal models of fasting rats, which mimicked the decreased glucose uptake/metabolism observed in brains of individuals with Alzheimer disease (AD), we found a decrease in O-GlcNAcylation and increase in phosphorylation of NF-M. We also observed decreased O-GlcNAcylation and an increased phosphorylation of NF-M in AD brain. These results suggest that O-GlcNAcylation and phosphorylation of NF-M are regulated reciprocally and that the hyperphosphorylation and accumulation of NF-M in AD brain might be caused by impaired brain glucose uptake/metabolism via down-regulation of NF-M O-GlcNAcylation.