Synaptic activation in the presence of competitive (D,L-APV, CNQX) and noncompetitive (MK-801, GYKI-52466) ionotropic glutamate receptor antagonists induced fast (1090%, rise time of 15-30 msec) postsynaptic responses in CA3 pyramidal neurons from acute and cultured hippocampal slices. Postsynaptic currents were studied extensively in slice cultures, and displayed a linear current-voltage relationship, with a reversal potential between 0 mV and +10 mY, suggesting the activation of a nonselective cationic conductance. Inhibition of the GTPase cycle by intracellular perfusion with the nonhydrolyzable analog of GDP, GDPβS, blocked the fast postsynaptic responses evoked in ionotropic antagonists, as well as baclofen-mediated outward K+ currents, known to be mediated by G protein-coupled GABA(B) receptors. Intracellular perfusion with GDPβS did not affect the AMPA/kainate component of the synaptic currents. Irreversible activation of G proteins by intracellular perfusion with the nonhydrolyzable analog of GTP, GMP-PNP, occluded the baclofen responses, and evoked an inward current, consistent with the synaptically mediated conductance. Incubation of the slice cultures in pertussis toxin for 72 hr blocked baclofen-induced outward K+ currents, while the fast postsynaptic currents remained. The metabotropic glutamate receptor (mGluR) agonists 1S,3R-ACPD and 1S,3S-ACPD induced an inward current in the presence of the ionotropic antagonists, and occluded the fast EPSCs. The fast EPSCs were partially blocked by the mGluR antagonists L-AP3 and (+)MCPG, but there was differential antagonist sensitivity in two pathways stimulated (CA3 stratum radiatum vs CA3 stratum oriens). These data suggest that fast postsynaptic responses evoked in the presence of ionotropic glutamate receptor antagonists are mediated by G protein-coupled mGluRs linked to nonselective cationic channels.