The effects of CGP 37849 [DL-(E)-2-amino-4-methyl-5-phosphono-3-pentenoate] and its ethylester CGP 39551 on whole-cell currents evoked by the endogenous excitatory amino acids, L-glutamate and L-aspartate, were studied in cultured mouse spinal cord neurones. Although CGP 37849 was the more potent compound, both antagonists inhibited 20 microM L-aspartate or 2 microM L-glutamate currents concentration-dependently and reversibly. We calculated IC50 values of 370 +/- 180 nM for CGP 37849 and 2200 +/- 140 nM for CGP 39551 (inhibition of L-aspartate current), and 210 +/- 25 nM for CGP 37849 and 6000 +/- 4700 nM for CGP 39551 (inhibition of L-glutamate current). Both CGP 37849 and CGP 39551 selectively blocked N-methyl-D-aspartate (NMDA)-evoked inward current. Current evoked by 5 microM kainate or 5 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) was unaffected by 10 microM CGP 39551. Current evoked by NMDA was concentration-dependently blocked by CGP 39551 with an IC50 of 2100 +/- 220 nM. After application of 10 microM CGP 37849, 17 +/- 6% of the current evoked by 5 microM L-glutamate remained. This residual current was due to non-NMDA receptor activation since application of 25 microM 2-amino-5-phosphonovalerate (APV) together with CGP 37849 did not significantly alter the residual current, whereas application of 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) with CGP 37849 did significantly inhibit this current. Clamping cells at potentials ranging from -80 to +60 mV showed a linear potential--current relationship for the 20 microM L-aspartate-evoked current with reversal potential around 0 mV. The proportion of the L-aspartate current antagonized by CGP 37849 or CGP 39551 appeared to be independent of clamping potential. The concentration--current relationship of L-aspartate in the absence of the antagonists showed an EC50 of 49 +/- 14 microM. Upon application of 1 microM CGP 37849 and 10 microM CGP 39551, the L-aspartate concentration--current curve shifted to higher concentrations, and resulted in a 5- and 13-fold increase in the EC50 of L-aspartate, respectively, whereas Imax was not changed by application of the antagonists. Thus, the potent NMDA antagonists CGP 37849 and CGP 39551 were shown to inhibit excitatory amino acid responses specifically by competitive binding to the neurotransmitter recognition site of the NMDA receptor. Selective, competitive antagonism of L-glutamate- and L-aspartate-evoked NMDA receptor responses probably underlies the effects of CGP 37849 and CGP 39551 such as their anticonvulsant, neuroprotectant and antidepressant actions.