The potencies of various N-methyl-D-aspartate(NMDA) receptor channel blockers were determined at recombinant NMDA receptors containing differing combinations of NR1 and NR2 subunits expressed in Xenopus laevis oocytes. When the NR1 subunit was varied (NR1e/NR2A or NR1b/NR2A), none of the 9 channel blockers tested displayed a statistically different affinity. In contrast, altering NR2 composition changed the affinities of several channel blockers. Three of 10 compounds displayed significantly higher affinities for NR1b/NR2C receptors than NR1b/NR2A receptors, and three of five compounds had higher affinity at NR1b/NR2C than NR1b/NR2B receptors. Both MK-801 and N-[1-(2-thienyl)cyclohyxyl]piperidine displayed identical affinities at all receptor subunit combinations tested. However, these two compounds displayed significantly slower rates of blockade and unblockade at NR1b/NR2C than at NR1b/NR2A receptors, perhaps reflecting the shorter mean open times of NR1/NR2C receptors. NR1b/NR2B and NR1b/NR2A were distinguished by one of five compounds tested. Taken together, these results indicate that NR2 subunits impart differing pharmacological profiles to NMDA receptors; thus, it may be possible to develop NMDA receptor channel blocker antagonists of greater subtype selectivity.