Under pathological conditions, nitric oxide (NO) is presumed to be cytotoxic through its interaction with superoxide and the subsequent generation of peroxynitrite (Beckman et al., 1990; Proc. Natl. Acad. Sci. 87: 1620). However, numerous reports indicate that NO can limit tissue injury engendered by excess production of reactive oxygen species (ROS) (Wink et al., 1993; Proc. Natl. Acad. Sci. 90: 9813). As ROS may be important downstream mediators of excitoxic neuronal death, we set out to determine the effect of NO donor compounds on N-Methyl-D-Asparate (NMDA) induced neuronal injury in mixed murine cortical cell cultures and whether this effect could be correlated with the amount of NO produced. Brief intense exposure to NMDA(200μM; 5 min) resulted in 60-80% neuronal cell death within 24 hr. Freshly prepared NONOate-type NO donors, DEA/NO, SPER/NO and PAPA/NO (30 -3000 μM), given at the same time as NMDA, concentration-dependently decreased neuronal injury as assessed by the reduction in the amount of lactate dehydrogenase released from neurons into the external medium. However, Concentrations of NONOates that were found to protect against excitotoxicity were neurotoxic when exposure was extended beyond two hours even in the absence of NMDA. In contrast another NO donor SIN-1, exacerbated NMDA-induced injury but was not toxic to cells following prolonged exposure. In each case, the neuroprotection and neurotoxicity were lost when donors were made one day prior to experimentation and allowed to “decay,” i.e. liberate their NO. Further, NONOates produced abundant amounts of nitrite, a stable product of NO, (>50 nmol/5 mins) while SIN-1 produced very small amounts (< 5 nmols/ 5 mins), perhaps explaining the difference in the effect of these compounds. Present results suggest that fine tuned release of NO may be needed in order to derive the potential neuroprotective effects while avoiding neurotoxicity. (Supported, in part, by a grant from the PhRMA foundation to SJH).