The influence of intracellular pH (pHi) changes on the formation of inositol phosphate metabolites (IPs) produced by glutamatergic stimulation was studied in 8-day-old rat brain synaptoneurosomes. For this purpose pHi was measured using 2',7'-bis-(2-carboxyl)-5,6-carboxyfluorescein (BCECF) fluorimetric assay in parallel with the basal and receptor-mediated formations of inositol monophosphate (IP1) and inositol bisphosphate (IP2). We found that glutamate (1 mM), which induces a transient acidification (delta pH = -0.05), produces an identical accumulation of IP1 and IP2. K+ (30 mM), which provokes an alkalinization of the internal medium (delta pH = +0.22), mainly leads to the formation of IP1 metabolites. Paired combinations of glutamate with 1, 5 and 10 mM NH4+ finally result in an alkalinization of the intrasynaptoneurosomal medium. These combinations produce a strong decrease of the IP2 level concomitant with an increase of the IP1 formation, compared to the levels of IP1 and IP2 evoked by glutamate alone. The total amount of IPs (IP1 + IP2) produced by these combinations is not different from that obtained with glutamate alone. Paired combinations of carbachol with NH4+ produce an identical alkalinization to that produced by NH4+ alone. These combinations produce an increased IP1 accumulation, while the IP2 formation is slightly decreased. When the internal medium is acidified by diminishing the external concentration of Na+, the ratio IP1/IP2 produced after metabotropic glutamate receptor (mGluR) activation is shifted to lower values, while it is not affected for the muscarinic stimulation. These data suggest that the mGluR-associated pathway in synaptoneurosomes is sensitive to pHi shifts, while the muscarinic receptor-associated pathway is less altered when pHi is manipulated. It may be proposed that pH-sensitive inositol phosphate dephosphorylating systems, i.e. phosphatases, are associated with mGluRs in this preparation.