In the mammalian central nervous system, the N-methyl-D-aspartate (NMDA) subclass of glutamate receptors may play an important role in brain diseases such as stroke, brain or spinal cord trauma, epilepsy, and certain neurodegenerative diseases. Compounds which specifically antagonize the actions of the neurotransmitter glutamate at the NMDA receptor ion-channel site offer a novel approach to treating these disorders. CERESTAT (4, aptiganel CNS 1102) is currently undergoing clinical trial for the treatment of traumatic brain injury and stroke. Previously, we reported that analogues of N-1-naphthyl-N'-(3-ethylphenyl)-N'-methylguanidine (4) bound to the NMDA receptor ion-channel site with high potency and selectivity. Recently, molecules active at both sigma receptors and NMDA receptor sites were investigated. A series of substituted diphenylguanidines 6 which are structurally related to N-1-naphthyl-N'-(3-ethylphenyl)-N'-methylguanidine was prepared. Compounds containing appropriate substitution pattern in one of the phenyl rings of diphenylguanidines displayed high affinity. For example, N-(2,5-dibromophenyl)-N'-(3-ethylphenyl)-N'- methylguanidine (27b, R2 = R5 = Br, R3 = C2H5) exhibited potency at both sigma receptors and NMDA receptor sites; 27b also showed high efficacy in vivo in a neonatal rat excitotoxicity model. Further studies indicated that substituent effects were important in this compound series, and 2,5-disubstituted phenyl was the preferred substitution pattern for high-affinity binding at NMDA receptor sites. Bromo and methylthio were the optimal substituents for the R2 and R5 positions of the 2,5-disubstituted phenyl group, respectively. N-(2-Bromo-5-(methylthio)phenyl)-N'- (3-ethylphenyl)-N'-methylguanidine (34b, R2 = Br, R5 = SMe, R3 = C2H5) was highly active at NMDA receptor sites. We found that the binding affinity of guanidines of type 6 could be further enhanced with the appropriate substitution at R3. Optimal activity in this series are afforded by 43b and 44b (R2 = Cl or Br, R5 = R3 = SCH3). Both 43b and 44b bound to NMDA receptor sites with high potency and selectivity (Ki vs [3H]MK-801: 1.87 and 1.65 nM, respectively); these compounds are active in vivo in various animal models of neuroprotection. The structure--activity relationships for these compounds at the NMDA receptor ion-channel site are discussed.