Microcystin-leucine-arginine (MCLR) is the most common form of microcystins, which are environmental toxins produced by cyanobacteria, and its hepatotoxicity has been well-documented. However, the neurotoxic potential of MCLR remains to be further elucidated. In the present study, we investigated whether intracerebroventricular (i.c.v.) infusion of MCLR induces mortality and neuronal loss in the hippocampus of mice. Because we found that MCLR impairs memory function in the hippocampus at a low dose (4 ng/μl/mouse, i.c.v.) without a significant neuronal loss, we focused on this dose for further analyses. Results showed that MCLR (4 ng/μl/mouse, i.c.v.) significantly increased oxidative stress (i.e., malondialdehyde, protein carbonyl, and synaptosomal ROS) in the hippocampus. In addition, MCLR significantly increased superoxide dismutase (SOD) activity without corresponding induction of glutathione peroxidase (GPx) activity, and thus led to significant decrease in the ratio of GPx/SODs activity. The GSH/GSSG ratio was also significantly reduced after MCLR treatment. GPx-1 overexpressing transgenic mice (GPx-1 Tg) were significantly protected from MCLR-induced memory impairment and oxidative stress. The DNA binding activity of nuclear factor erythroid-derived 2-related factor 2 (Nrf2) in these mice was significantly enhanced, and the ratios of GPx/SODs activity and GSH/GSSG returned to near control levels in the hippocampus. Importantly, memory function exhibited a significant positive correlation with the ratios of GPx/SODs activity and GSH/GSSG in the hippocampus of MCLR-treated non-transgenic (non-Tg)- and GPx-1 Tg-mice. Combined, our results suggest that MCLR induces oxidative stress and memory impairment without significant neuronal loss, and that GPx-1 gene constitutes an important protectant against MCLR-induced memory impairment and oxidative stress via maintaining antioxidant defense system homeostasis, possibly through the induction of Nrf2 transcription factor.