Recent reports suggest that NMDA receptor antagonists when administered in vivo can protect dopaminergic neurons from the toxic actions of MPP+. In the present study the possible neuro-protective effects against MPP+ toxicity of the noncompetitive NMDA receptor antagonist MK-801 was studied in primary cultures of fetal rat mesencephalic dopamine neurons. MK-801 failed to protect dopaminergic neurons from MPP+ toxicity at concentrations that completely block NMDA-induced toxicity of these same neurons. In contrast to work carried out in cerebellar granule cells, MPP+ toxicity of mesencephalic dopamine neurons was unaffected by preexposure to subtoxic concentrations of either NMDA or cycloheximide. Our findings suggest that the toxic effects of MPP+ on dopaminergic neurons are not mediated through a direct interaction with the NMDA subtype of glutamate receptor.