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 (I-IFS) 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 mu M) did not inhibit the induction of LTP, but did prevent the block of LTP by DCG-IV. MCCG (500 mu M) strongly inhibited the induction of LTD. DCG-IV (50 nM) did not inhibit LTD induction.