The effect of ethanol (EtOH) on synaptic transmission mediated by N- methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors was investigated in slices from adult rat hippocampus. Synaptic responses were elicited by stimulation of stratum radiatum and were recorded in CA1 stratum radiatum or stratum pyramidale. Population EPSPs (pEPSPs) mediated by NMDA receptor activation were isolated by application of a solution containing the kainate/quisqualate receptor antagonist 6,7- dinitroquinoxaline-2,3-dione and either low (0.1 mM) Mg2+ or 100 microM bicuculline. Increasing concentrations of EtOH produced increasing inhibition of NMDA receptor-mediated pEPSPs with EtOH concentrations between 1 and 50 mM. At a concentration of 50 mM, EtOH inhibited NMDA receptor-mediated pEPSPS by 43%; the inhibition by 100 mM EtOH was not significantly different from that produced by 50 mM. Methanol and 1- butanol also inhibited the NMDA receptor-mediated pEPSPs; the potency of the alcohols for inhibition of NMDA receptor-mediated pEPSPs was 1- butanol greater than ethanol greater than methanol. pEPSPs mediated by non-NMDA glutamate receptors were isolated by the application of the NMDA receptor antagonist d,1–2-amino-5-phosphonovaleric acid in the presence of 1.5 mM Mg2+. These pEPSPs were not significantly affected by 50 mM EtOH, whereas 100 mM EtOH reduced the amplitude of these pEPSPs by 9%. The observations indicate that synaptic excitation mediated by NMDA receptors in tissue from adult rat is inhibited by intoxicating concentrations of EtOH. The data are consistent with the hypothesis that EtOH-induced inhibition of EPSPs mediated NMDA receptors may contribute to the intoxicating effects of EtOH.