Both hypoxia and bilirubin are common risk factors in newborns, which may act synergistically to produce anatomical and functional disturbances of the CNS. Using primary cultures of neurons from the fetal rat brain, it was recently reported that neuronal apoptosis accounts for the deleterious consequences of these two insults. To investigate the influence of hypoxia, bilirubin, or their combination on the outcome of neuronal cells of the immature brain, and delineate cellular mechanisms involved, 6-d-old cultured neurons were submitted to either hypoxia (6 h), unconjugated bilirubin (0.5 M), or to combined conditions. Within 96 h, cell viability was reduced by 22.7% and 24.5% by hypoxia and bilirubin, respectively, whereas combined treatments decreased vital score by 34%. Nuclear morphology revealed 13.4% of apoptotic cells after hypoxia, 16.2% after bilirubin, and 22.6% after both treatments. Bilirubin action was specifically blocked by the glutamate receptor antagonist MK-801, which was without effect on the consequences of hypoxia. Cerebral hypoxia remains a significant risk for neonatal morbidity and mortality, as well as long-term neurologic sequelae such as cerebral palsy, mental retardation, or seizure disorders (1, 2). Although lack of oxygen has been associated with neuronal degeneration (3, 4), it was not clear whether hypoxia itself, without the influence of combined ischemia, kills brain cells. In contrast to studies in intact animals, which are often difficult to interpret due to a number of confounding variables, cell culture seems a useful tool for elucidating cellular and molecular mechanisms involved in clinical disorders such as cerebral hypoxia (5, 6). Indeed, neuronal cells cultured in chemically defined medium have proven to mimic morphologic, electrophysiological, and neurochemical development and differentiation occurring in vivo (7,8), and such a preparation provides the advantage of investigating cellular responses inherent to a single variable. By using cultured neurons from the developing rat forebrain as a model, we have previously demonstrated that transient oxygen deprivation may trigger neuronal death through an apoptotic process that requires specific and time-dependent changes in protein synthesis (9, 10), a conclusion supported by other studies (6, 11).Bilirubin constitutes another potential cause of brain injury (12, 13). Hyperbilirubinemia is still a common concern in neonatology, especially in the care of low birth weight prema- ABSTRACT 507