Tricyclic quinoxalinediones: 5,6-dihydro-1H-pyrrolo[1,2,3-de] quinoxaline-2,3-diones and 6,7-dihydro-1H,5H-pyrido[1,2,3-de] quinoxaline-2,3-diones as potent antagonists for the glycine binding site of the NMDA receptor.