Group II metabotropic glutamate receptors (mGluRs) play an important role in the regulation of hippocampal synaptic plasticity in vivo: long-term potentiation (LTP) is inhibited and long-term depression (LTD) is enhanced by activation of these receptors. The contribution, in vivo, of the individual group II mGluR subtypes has not been characterized. We analysed the involvement of the subtype mGluR3 in LTD and LTP. Rats were implanted with electrodes to enable chronic measurement of evoked potentials from medial perforant path-dentate gyrus synapses. Neither the selective mGluR3 agonist, N-acetylaspartylglutamate (NAAG), nor the antagonist beta-NAAG, given intracerebrally, affected basal synaptic transmission. beta-NAAG significantly inhibited LTD expression. NAAG exhibited transient inhibitory effects on the intermediate phase of LTD. Whereas NAAG altered paired-pulse responses, beta-NAAG had no effect, suggesting that antagonism of mGluR3 prevents LTD via a postsynaptic mechanism, whereas agonist activation of mGluR3 modulates LTD at a presynaptic locus. NAAG impaired the expression of LTP, whereas beta-NAAG had no effect. NAAG effects on LTP were blocked by EGLU, a selective group II mGluR antagonist. Our data suggest an essential role for mGluR3 in LTD, and a modulatory role for mGluR3 in LTP, with effects being mediated by distinct pre- and post-synaptic loci.