The role of cGMP in the mediation of glutamate neurotoxicity remains controversial. Some reports indicate that cGMP mediates glutamate neurotoxicity while others indicate that cGMP is neuroprotective. We have studied the role of cGMP in the mediation of glutamate and nitric oxide neurotoxicity in primary cultures of cerebellar neurons. Inhibition of soluble guanylate cyclase prevents glutamate and nitric oxide neurotoxicity. There is a good correlation between inhibition of cGMP formation and neuroprotection. Moreover 8-Br-cGMP, a cell permeable analog of cGMP, induced neuronal death. These results indicate that increased intracellular cGMP is involved in the mechanism of neurotoxicity. Inhibitors of phosphodiesterase did not increase intracellular cGMP but increased the content of cGMP in the extracellular medium and prevented glutamate neurotoxicity. Moreover, addition of cGMP to the extracellular medium also prevented glutamate neurotoxicity in cerebellar neurons in culture. These results are compatible with a neurotoxic effect of increased intracellular cGMP and a neuroprotective effect of increased extracellular cGMP.