In established 8-12-day-old primary cultures of differentiated rat cerebellar granule neurons the level of metabotropic glutamate receptor 1 (mGluR1) mRNA and the sensitivity of cultures to the agonist-stimulated inositol phosphate (IP) formation was reversibly modified by changing the depolarizing properties of the medium, i.e. the medium KCl concentration. The mGluR1 mRNA content was suppressed by increasing the medium KCl content and elevated by decreasing it. The mGluR agonist quisqualate inhibited the mGluR1 expression. This is the first direct demonstration of a differential expression of neuronal mGluR1 in an established neuronal culture. The model can be used to study the molecular mechanism of neuronal plasticity.