Hypercholesterolemia has been suggested as a risk factor for Alzheimer's disease (AD). A genetic risk factor for AD is the E4 allele of apolipoprotein E (apoE). ApoE is the major lipoprotein transporter in the brain, and is mainly produced by glial cells. The present study is focussed on analysing the effects of high cholesterol (HC) diet, duration 9 months, on glial activation in the brain, both in wild type (WT) mice and in mice with a null mutation in the apoE gene (knock-out, KO) mice. The activation of astrocytes and microglia was analysed after immunohistochemical labelling of glial fibrillary acidic protein (GFAP), and F4/80, respectively. In addition, the expression of the antioxidant enzyme NAD(P)H:quinone oxidoreductase (NQO1) was analysed. There was a marked stimulation of astrocyte and microglial activation as well as induced expression of NQO1 in the hippocampus and cerebral cortex upon HC diet. Furthermore, there was significant astrocyte activation in the apoE KO mice, as compared to the WT mice, on ND. The long time exposure to HC diet combined with apoE deficiency resulted in a synergistic effect on the expression of NQO1 in the brain.