Particular subtypes of metabotropic glutamate receptors (mGluRs) have been shown to be specifically involved in certain types of long-term synaptic plasticity and learning. We examined whether inhibition of mGluR5 by the specific noncompetitive antagonist 2-methyl-6-(phenylethynyl)-pyridine (MPEP) has any functional consequences on long-term potentiation in the dentate gyrus in vivo and on learning of a spatial alternation task. Intracerebroventricular application of 13.8 microg MPEP 30 min before tetanization resulted in a rapid decline of potentiation during the first 7 min and a significantly lower potentiation of the MPEP group as compared to controls. The same dose of the antagonist given 30 min before training of a Y-maze spatial alternation task caused a marked impairment of retention tested 24 h later. In contrast, MPEP had virtually no effects on retention if injected immediately after the training session. Our findings suggest an important function of mGluR5 during the initiation of synaptic plasticity and memory formation.