Glutamate has been shown to function as a toxic agent in neuronal and glial cells, as well as an excitatory neurotransmitter throughout the central nervous system. In the present study, we examined the effect of increasing glutamate concentration on the induction of apoptosis in the two human glioblastoma cell lines GB-4 and GB-12. Glutamate exposure caused cell death of GB-4 and GB-12 in a dose-dependent manner. The cells were found to die via apoptosis in response to glutamate based on the following criteria: propidium iodide (PI) staining, H-E staining, electron microscopic analysis, and the TdT-mediated dUTP-biotin nick end labeling (TUNEL) method. The glutamate-induced apoptosis appears to involve the modulation of Bcl-2 family gene products such as Bcl-2, Bcl-xL, and Bax-alpha. Both Bcl-2 and Bcl-xL were down-regulated by glutamate at 24 h and further at 48 h. The apoptosis-promoting product p21 Bax-alpha was also down-regulated in GB-12 but slightly up-regulated in GB-4, accompanied by generation of variant form of p18 Bax-alpha in both cell lines. These findings suggest that glutamate toxicity results in cellular death via an apoptotic mechanism which appears to involve the Bcl-2/Bax-alpha molecular complex.