3-Substituted-2-quinolones (6-6) have been identified as glycine-site N-methyl-D-aspartate receptor antagonists. It is proposed that the a-phenyl lactam unit in the potent 4-hydroxy-3-phenyl derivatives (7d and 8b, L-701,31 5) may act as a glycine bioisostere in receptor recognition.Antagonists of the N-methyl-D-aspartate (NMDA) subtype of excitatory amino acid receptor have aroused considerable interest as potential agents for the treatment of central nervous system disorders such as cerebral ischaemia, head injury, epilepsy, schizophrenia and Parkinson's disease.1 Rapid progress has been made in the development of antagonists which act at the glycine modulatory site associated with the NMDA receptor, but existing compounds, including kynurenic acid derivatives (la-c) 2, 2-carboxy tetrahydroquinolines (2, L-689,560),3 quinoxaline-2,3-diones (3),4 and a-amino hydroxamates (4, L-687,41 4),5 are limited either by poor in vivo properties (l-3)617 or highly restrictive structure-activity requirements (4).8One approach we have explored to identify alternative glycine antagonists has focussed on replacement of the acidic 2-carboxyl in 1 and 2 with suitable bioisosteric groups. In this report we show that placing an acidic moiety within the heterocyclic ring of 2-quinolone derivatives leads to potent glycine antagonists. It is suggested that these compounds, exemplified by the highly active 4-hydroxy-3-phenyl-2-quinolones 7d and 8b, contain a novel amino acid bioisostere.