The effects of various anthraquinone polyamines (AQP) were studied at recombinant N-methyl-d-aspartate (NMDA) receptors expressed in Xenopus laevis oocytes. The AQP derivatives had different numbers of methylene groups between the NH(2) (or NH) groups in their spermidine-like tail. Thus, we termed these derivatives AQ33, AQ34, etc. All AQP derivatives inhibited responses of NR1/NR2 receptors in oocytes voltage-clamped at -70 mV, with IC(50) values between 4 and 22 microM. The block was strongly voltage-dependent. AQ34 and AQ33b inhibited responses of NR1/NR2 receptors but did not inhibit responses of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors expressed from GluR1 or GluR2(Q), indicating that AQ34 and AQ33b are preferential NMDA antagonists. Results of experiments using mutant NR1 and NR2 subunits identified residues that influence block by AQ34 and AQ33b. These residues are located in the outer vestibule at the selectivity filter/narrowest constriction of the channel and in the inner vestibule below the level of the selectivity filter. The results with mutant NR1 and NR2 subunits are consistent with the idea that NR1(Asn616) and NR2B(Asn616), but not NR2B(Asn615), make the narrowest constriction of NMDA channel.