Excessive activation of glutamate receptors may contribute to neuronal loss after a traumatic or ischemic central nervous system insult. Such injuries are often associated with hemorrhage and extravasation of hemoglobin, a prooxidant and putative neurotoxin. In this study, we investigated the effect of nontoxic concentrations of hemoglobin on the neurotoxicity of the synthetic glutamate receptor agonists NMDA, AMPA, and kainate in primary murine cortical cultures. Continuous exposure to each excitotoxin alone for 24-28 h produced concentration-dependent neuronal death (EC(50) about 12 mu M for AMP(+)A, 50 mu M for kainate, and 12 mu M for NMDA). Hemoglobin 0.25-1.0 mu M consistently potentiated the neurotoxicity of low concentrations of AMPA and kainate, increasing neuronal loss by about 150% at 6 mu M AMPA and by about 90% at 30 mu M kainate. This effect was attenuated by the iron chelator deferoxamine and the alpha-tocopherol analogue trolox. Hemoglobin coexposure had less impact on slowly triggered NMDA neurotoxicity, significantly increasing neuronal death only at agonist concentrations that alone produced little or no injury. Hemoglobin pretreatment had no effect on the rapidly triggered excitotoxicity induced by brief exposure to high concentrations of NMDA. These results suggest that hemoglobin may contribute to neuronal loss after CNS hemorrhage by exacerbating excitotoxicity. At moderate levels of agonist exposure, this effect may be somewhat selective for the AMPA/kainate component of injury.