Glutamate stimulates phosphatidyl inositol hydrolysis and mobilizes intracellular calcium through the mediation of metabotropic glutamate receptors (mGluRs), in particular the "Group I" receptors mGluRs, mGluR1, and mGluR5. This activity is markedly enhanced in developing brain relative to the adult. To determine whether this may be due to an increased amount of mGluR5 present in the developing brain, we examined mGluR5 expression using western blotting to measure mGluR5 protein reverse transcription polymerase chain reaction (RT-PCR) to measure mGluR5 mRNA, and immunocytochemistry to assess the regional distribution of mGluR5 morphologically. Western blotting revealed that in all brain regions examined there is more mGluR5 protein present in developing brain than in the adult. In most regions, the developmental decrease was over two-fold. Total mGluR5 mRNA also decreased with development in most regions, but to a much lesser extent than the protein, suggesting that there is considerable post-transcriptional regulation of the expression of this receptor. RT-PCR analysis also demonstrated that in most regions the mGluR5a splice variant is most abundant in the young animals but mGluR5b predominates in the adult. Light microscopic immunocytochemistry indicated that expression is widespread in developing brain, and that the developmental decrease in receptor concentration is due to both an increased growth of receptor-poor tissue regions and decreased expression within receptor-rich regions.