Oligodendrocytes (OL) are considered to be the target of the autoimmune inflammation in the central nervous system (CNS) of multiple sclerosis (MS) patients. Recently, statins, which are widely used lipid-lowering drugs, were found to have beneficial effects both in MS patients and in an animal model of MS. However, neither the exact mechanisms of action of statins in the CNS are known, nor their effects on the myelin-forming OL. The aim of this in vitro study is to investigate whether simvastatin, a frequently-used, lipophilic statin, affects OL viability under physiological conditions as well as during oxidative stress (OS), which is known to play a major role in the pathogenesis of MS. OLN-93 cells, immortal oligodendroglial cells derived from primary rat brain glial cultures, were grown on uncoated glass coverslips in Dulbecco's Modified Eagle Medium with 10% fetal calf serum (FCS). Immunocytochemical and immunocytofluorescence experiments revealed that OLN-93 cells express markers characteristic for OL precursors (A₂B₅, NG2) as well as mature OL markers (CNP, PLP/DM20), when cultured under these conditions. OS was generated via H₂O₂ treatment. Cell viability and the cellular cholesterol and protein content were determined via colorimetric assays, respectively the 3-4,5-dimethylthiazol-2-yl-2,5-difenyl tetrazoliumbromide (MTT)-assay, the Elitech cholesterol SL kit and the Bradford assay. Flow cytometric analyses were performed (propidium iodide / calcein-acetoxymethyl ester) to distinguish between apoptotic and necrotic cell death. Simvastatin treatment (50 mM, 48 h) does not alter the cholesterol content of confluent OLN-93 cells. However, it reduces cellular cholesterol in subconfluent cells by approximately 50 %, similar to the effect of the cholesterol-lowering agent methyl-b-cyclodextrine (mbCD, 5 mM, 1 h). Simvastatin addition during 48 h induces apoptosis in subconfluent OLN-93 cells in a dose-dependent way (for 50 mM simvastatin: 46% apoptosis and for 1 mM: 23%), whereas no detectable loss of cell viability is observed after 16 h. mbCD-treated OLN-93 cells remain viable. The reduction in viability of H₂O₂-treated (3 h, 50 mM) OLN-93 cells does not change neither by pre-incubation (50 mM, during 16 h before H₂O₂ treatment) nor by co-incubation (50 mM, 3 h simultaneously with H₂O₂) with simvastatin. Our data suggest that effects of simvastatin on oligodendroglial cholesterol levels depend on the activity of the cholesterol metabolism of the cells, since simvastatin lowered cholesterol only in proliferating subconfluent cells. Moreover, the simvastatin-induced blockage of cholesterol synthesis triggers apoptosis in subconfluent OLN-93 cells. Finally, it can be concluded that shorter incubation periods with simvastatin -which induce no cellular damage as such- do not improve the antioxidant defense of OLN-93 cells to OS, in contrast to the known statin-induced upregulation of endogenous antioxidants in endothelial cells.