The effects of the metabotropic glutamate receptor (mGluR) group II agonist (2S,1'R,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG-IV) and the antagonist 2S,1S',2S'-2-methyl-2-(2'-carboxycyclopropyl)glycine (MCCG) on the induction of long-term potentiation (LTP) and long-term depression (LTD) of the field excitatory postsynaptic potential (epsp) in the dentate gyrus of the rat hippocampus were investigated in vitro. In control media, high frequency stimulation (HFS) consisting of a series of trains of stimuli at 200 Hz induced LTP, and low frequency stimulation (LFS) consisting of 900 stimuli at 1 Hz induced LTD. DCG-IV (50 nM) strongly inhibited the induction of LTP. The block of LTP induction was not caused by a frequency-dependent inhibition of the epsp during HFS, as the epsps evoked during HFS were not altered by DCG-IV. MCCG (500 microM) did not inhibit the induction of LTP, but did prevent the block of LTP by DCG-IV. MCCG (500 microM) strongly inhibited the induction of LTD. DCG-IV (50 nM) did not inhibit LTD induction.