Long-term potentiation in the dentate gyrus was studied in vivo in mutant mice lacking the metabotropic glutamate (mGlu) subtype 1 receptor. Long-term potentiation was induced in wild type animals (n = 7), but was reduced in mice lacking this receptor subtype (n = 7). These data suggest that mGlu receptors may be involved in the induction or maintenance of synaptic plasticity or that these receptors are required during development to establish this plasticity.