In several neurological disorders including Alzheimer's disease, abnormal accumulations of cytoskeleton-associated proteins manifest as neurofibrillary tangles (NFTs) in vulnerable brain regions. Antibodies recognizing tau (5E2 and Alz-50) and ubiquitin epitopes in NFTs were used to examine the influence of glutamate and Ca2+ influx on antigen expression in cultured rat hippocampal neurons. Glutamate caused the degeneration of a subpopulation of pyramidal neurons, which exhibited increased immunostaining with all three antibodies. Subtoxic levels of glutamate also increased 5E2 and Alz-50 antigen levels in a subpopulation of neurons, particularly in the distal regions of the axons. Both glutamate-induced degeneration and increases in tau and ubiquitin immunostaining were prevented by removal of extracellular Ca2+. Increased immunostaining was also induced by Ca2+ ionophore A23187 or elevated levels of extracellular K+. The antigenic changes occurred within 1 hr of exposure to glutamate or A23187 and were not prevented by the protein synthesis inhibitor cycloheximide. These data indicate that Ca2+ influx caused by glutamate can lead to modifications of extant proteins similar to those seen in NFTs.