Selenium (Se) is a very effective anti-cancer agent. We studied the effects of inorganic Se compounds on induction of apoptosis by which Se compounds exert cancer chemopreventive activity. With the use of HSC-3 human oral squamous cell carcinoma cells, the present study showed that treatment with Se for 72 h, in the form of SeO2 and Na2SeO3, but not Na2SeO4, markedly induced apoptosis in a dose-dependent manner. Treatment of HSC-3 cells with 100 microM SeO2 resulted in the caspase-3-like and -9-like activation. Se compounds induced a loss of mitochondrial membrane potential (DeltaPsim), but did not induce the generation of reactive oxygen species. Treatment with SeO2 for 18 h resulted in 80% loss of reduced glutathione (GSH), which is known to be involved in the metabolism of Se. Treatment with N-acetyl-L-cysteine, or exogenous GSH, prevented the SeO2-induced apoptosis. Treatment with GSH led to the partial reverse in reduction of DeltaPsim caused by SeO2, while buthionine sulfoximine augmented the SeO2- or Na2SeO3-induced apoptosis. These results suggest that modulation of the mitochondrial redox equilibrium by Se contributes to the mitochondrial pathway, regulating caspase-9-mediated apoptosis without a concurrent increase in ROS.