Phosphorylation of the astrocytic marker protein glial fibrillary acidic protein (GFAP) in hippocampal slices from immature rats is stimulated by glutamate agonists via a metabotropic receptor. In this study we investigated the modulation of this stimulation by guanine nucleotides. Recent work has shown that guanine nucleotides inhibit the binding of kainate to its receptors in a manner independent of G proteins. Gpp(NH)p, GDP-beta-S and GMP inhibited by approximately 50% the stimulation of GFAP phosphorylation by glutamate or 1S,3R-ACPD. In the case of glutamate and Gpp(NH)p it was shown that the inhibition was dose dependent. These results indicate that guanine nucleotides can inhibit glutamate-stimulated phosphorylation responses by interaction with a cell surface metabotropic receptor.