The mRNA for the type IV metabotropic glutamate receptor (mGluR4) is most prominently expressed in cerebellar granule cells, the olfactory system, the lateral septal nucleus, and most thalamic nuclei, but lower amount of the mRNA is found in many different brain regions. Agonist stimulation as studied in transfected cells was found to inhibit forskolin-stimulated cyclic-adenosine-monophosphate formation through a pertussis toxin-sensitive mechanism. Agonist rank order of potencies was: L-2-amino-4-phosphonobutyrate (L-AP4) > glutamate > (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (t-ACPD) > quisqualate > L-homocysteate = ibotenate. The results suggests that the mGluR4 may be responsible for many, but not all, effects of L-AP4.