A series of tricyclic quinoxalinediones, 5, 6-dihydro-W-pyrrolo [l, 2, 3-delquinoxaline-2, 3- diones and 6, 7-dihydro-lH, 5H-pyrido [1, 2, 3-de] quinoxaline-2, 3-diones, were synthesized and was evaluated for their affinity for the glycine binding site of the NMDA receptor using a [3H]-5, 7-dichlorokynurenic acid binding assay. The six-membered ring-fused tricyclic quinoxalinedione 18g (Ki= 9.9 nM) displayed high affinity for the glycine site. The anilide ...