In the last few years, there has been an important increase in interest in nitric oxide (NO) as an intercellular messenger, and its putative role in numerous CNS functions is being continually updated. Arginine, the nitric oxide precursor, has been found in our laboratory to be released following stimulation of the white matter in the cerebellum and of sensory afferents in the thalamus. Since arginine is localized in glial cells while the nitric oxide synthesizing enzyme is localized in different cells (predominantly in neurons), these findings may represent a transfer of arginine from glia to neurons in order to supply the nitric oxide synthase with its substrate. The mechanism underlying this glial-neuronal interaction seems to involve the activation of excitatory amino acid receptors present on glial cells. Our results speak for an intense crosstalk between neurons and glia (activation of glial receptors by neurotransmitter released from neurons) and between glia and neurons (supply of the nitric precursor arginine from glia to neurons). The form in which NO is released from cells has been much debated. The chemical identity of the endothelial-derived relaxing factor in particular is still a matter of dispute, the major contender being NO. and a S-nitrosothiol compound. Based on the strong reactivity of NO for thiols and on the presence of cysteine and glutathione at the mM level intracellularly and microM level extracellularly, we have investigated whether S-nitrosothiols, i.e. S-nitrosoglutathione, may be the potential "package" form in which NO could be stored. We demonstrated, with HPLC coupled to mass spectrometry techniques, the presence of endogenous nitrosoglutathione in rat brain tissue. This packaging of NO in the form of nitrosothiols might serve to facilitate its transfer, prolong its life, and target its delivery to specific effectors. That could confer a specificity of action to the widely diffusable messenger NO, may determine the range of effectiveness of NO and mitigate its adverse cytotoxic effects.