Reduced glutathione (GSH) is a low molecular weight thiol and the most abundant non-enzymatic antioxidant in the cell. Its property to maintain cellular thiol/disulfide redox state gives it a central place in the control of great diversity of vital cell processes including cell proliferation (1). Our group has recently investigated the possible relationship between telomerase activity and cellular GSH concentration and we have demonstrated that the peak of total GSH (tGSH) in 3T3 fibroblasts coincides with the peak of telomerase activity (TA) at 24h in culture preceding the exponential phase of the cell growth (2). The objective of the present work was to analyse the relation between GSH levels and cell cycle in a non-proliferative cell model. We chose rat embryonic neurons that once diferentiated do not have mitotic activity. Primary cultures were established of neurons from the cerebral cortex of fetal Wistar rats (gestation day 14) (3). To prevent non-neuronal proliferation 20 µM cytosine arabinoside (AraC) was added on the 4th day of culture, and 24h afterwards half of the culture medium was changed. Until the 18th day of culture we studied the cell cycle by flow cytometry, GSH levels by spectrofotometry and cell viability by double staining propidium iodide (PI; 2µg/ml) to identify dead cells and Hoechst (2µg/ml) to localize all the nuclei. The level of glial contamination was defined by inmunocitochemistry and was less than 3%. In the first 4 days of culture, before the AraC was added, the peak of GSH at 48h (39.6±4.6 nmol/mg prot, p<0.01 and p<0.05 vs 3h and 7 days, respectively) was followed by the maximum proliferation at 72h (S+M/G2=15±1.1, p<0.01 vs. 5.7±0.3 at 7 days). After the AraC was added, the cell culture stabilized in G0/G1 (39.2± 0.7%) and the level of GSH dropped significantly (27.9±2.5 nmol/mg prot at 7 days). A higher percentage of cell death could be attributed to the elimination of non-neuronal cells by AraC. Our results show that while freely proliferating, the primary culture of cerebral cortex demonstrates the same tendency as in 3T3 fibroblasts (2). The GSH maximum level preceeds the peak of proliferation. However, in a 97% pure neuronal culture, when the non-neuronal cells were eliminated, the level of GSH and proliferation is significantly lower and it remains stable throughout the period of culture.