Conventional intracellular recordings were obtained from layer II-III neurons in adult rat neocortical brain slices. Excitatory and inhibitory (I) postsynaptic potentials (PSPs) were evoked prior to and during bath application of agonists and antagonists of metabotropic glutamate receptors (mGluRs). In the presence of the selective mGluR agonist 1S,3R-1-aminocyclopentane-1,3- dicarboxylic acid (1S,3R-ACPD; 5– 200 microM), both excitatory and inhibitory components of the evoked PSPs were reversibly reduced. PSPs were significantly, but less effectively, decreased by L-2-amino-4-phosphonobutyric acid. Exposure to putative mGluR antagonists, alpha-methyl-4-carboxyphenylglycine or L- 2-amino-3-phosphonopropionic acid, did not inhibit the 1S,3R-ACPD- mediated effect. In the presence of 6,7-dinitroquinoxaline-2,3-dione and D-2-amino-5-phosphonovaleric acid, 1S,3R-ACPD reversibly depressed directly evoked neocortical IPSPs; however, quisqualic acid (1–10 microM) did not mimic this effect. Analysis of spontaneous PSPs and paired-pulse facilitation indicated a presynaptic locus of action for 1S,3R-ACPD at mGluRs. These findings indicate that a specific mGluR subtype(s) may modulate both excitatory and inhibitory synaptic transmission in the adult rat neocortex via a presynaptic reduction of transmitter release.