Previous research has suggested that repletion of cellular glutathione peroxidase (GPX1) activity by a single injection of Se was dissociated from the Se protection against the pro-oxidant-induced liver necrosis in Se-deficient rodents. Using the GPX1 knockout (GPX1-/-) mice, TUNEL assay, and apoptosis gene expression microarray, we have demonstrated strikingly different impacts of GPX1 knockout on hepatotoxicity and the related signaling induced by an intraperitoneal injection of 12.5 mg paraquat/kg body weight (b.wt.). In both Se-deficient GPX1-/- and wild-type (WT) mice, the paraquat did not induce typical liver necrosis, rather aponecrosis or necrapoptosis, a syncretic process of cell death sharing characteristics of both apoptosis and necrosis. The severity of liver aponecrosis and the associated mortality were reduced to a much greater extent by an injection of Se (ip, 50 microg/kg b.wt. as Na2SeO3) prior to paraquat stress in the WT mice, compared with the GPX1-/- mice. The induced liver aponecrosis seemed to be more apoptotic in the GPX1-/- mice but more necrotic in the WT mice. The paraquat-mediated gene or protein expression of proapoptotic Bax, Bcl-w, and Bcl-X(S), cell survival/death factors GADD45, MDM2, c-Myc, and caspase-3 was upregulated, but that of antiapoptotic Bcl-2 was downregulated in the GPX1-/- mice vs. the WT mice. Overall, these differences between the two groups of mice were related to a low level of liver GPX1 activity in the WT mice that represented < 4% of the normal physiological level. Therefore, the low level of GPX1 activity in the Se-deficient mice can exert a potent role in defending against liver aponecrosis induced by moderate oxidative stress.