We showed previously that a 6h-hypoxia followed by reoxygenation for 96h induces a delayed, mainly apoptotic cell death in cultured neurons from the fetal rat forebrain, whereas shorter hypoxia (3h) increases cell viability. To elucidate cellular events involved, the expression of specific proteins, either effector of apoptosis (Bax) or potentially related to cell recovery(Bcl-2, HSP70, HSP105), was analysed. After 6 days in vitro, neurons were exposed for 3 or 6h to hypoxia (95% N2-5% CO2) that decreased extracellular PO2 by 75-80%. Cells were then reoxygenated (95% air-5% CO2) for 96h. As a function of time, neuronal viability was assessed by the MTT procedure, while apoptosis was studied by the analysis of DNA integrity by gel electrophoresis and nuclear incorporation of the fluorescent dye DAPI. Protein synthesis was measured by incorporation of [3H]leucine, then neuronal proteins were separated by SDS-PAGE. Expression of specific proteins was investigated by immunohistochemistry and Western blot. All data reported are compared to controls kept in normoxia and are statistically significant(p<0.01). A 6h-hypoxia led to marked inductions of protein synthesis rates 1h after the onset of the insult (+77%, n=26 dishes) and 48h after reoxygenation (+72%, n=26). By 72h, synthesis rates decreased (-41%, n=24), concomitantly with reduced cell viability (-36%, n=30) and detection of 23% of apoptotic neurons (n=10). SDS-PAGE analysis showed specific alterations in the density of constitutive proteins during hypoxia and reoxygenation. Bax was overexpressed 1h after the onset of hypoxia, and then by 72 and 96h after reoxygenation. Bcl-2, HSP70 and HSP105 were also overexpressed at 1h, but decreased thereafter, with the development of apoptosis. Conversely, a 3h-hypoxia also led to increased protein synthesis after 1h, but not at 48h, and then to a late increase (+60%, n=10) by 72h post-reoxygenation, along with enhanced viability (+13%, n=10). During reoxygenation, Bax expression was not affected, while Bcl-2 and HSPs were overexpressed. Therefore, transient hypoxia induces early expression of specific proteins such as Bax, Bcl-2 and HSPs. Thereafter, depending on the duration of the insult, either apoptosis-related (Bax) or survival-related (Bcl-2, HSPs) proteins are overexpressed and may influence the final cell outcome.