The effect and mode of action of ethanol on N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors of rat cortical neurons in primary culture were compared by means of the patch-clamp technique. The maxima of the concentration-response curves for both NMDA and AMPA were markedly depressed by ethanol without an apparent shift of the curves to the right. Ethanol inhibited the effects of NMDA and AMPA concentration-dependently and equally well. Excitatory amino acid (EAA) currents were depressed to the largest extent when ethanol was continuously superfused during and between agonist applications; a smaller inhibitory effect was observed when ethanol was intermittently superfused during agonist applications only. There was no inhibition by ethanol, when its superfusion was between two agonist applications. According to expectations, the analysis of the ratios of plateau to peak currents failed to suggest a use-dependent blockade by ethanol. In addition, comparison of the voltage-current curves of NMDA and AMPA in the absence and presence of ethanol indicated a voltage-independent effect and no change in the reversal potential of the two agonists. Finally, the measurement of activation and deactivation time constants for the ethanol-induced inhibition of NMDA and AMPA responses confirms the failure of ethanol to cause an open-channel block. In conclusion, these findings as a whole indicate that ethanol inhibits NMDA and AMPA currents by a noncompetitive mechanism. The mode of action appears to be similar for both NMDA and AMPA; under the conditions of the present study, a selective interference with structural motifs of the NMDA receptor is unlikely.