N-(2 Naphthyl)-N’-(3-[I-125]-iodophenyl)-N’-methylguanidine ([I-125]-CNS 1261) was syn thesized as a potential radioligand to image N-methyl-D-aspartate (NMDA) receptor activation. [I-125]-CNS 1261 was prepared by radioiodination of N-(1 naphthyl)-N’-(3-tributylstannyphenyl)-N’-methylguanidine using (NaI)-I-125 and peracetic acid. [I-125]-CNS 1261 uptake in vivo reflected NMDA receptor distribution in normal rat brain, whereas in ischemic rat brain, uptake was markedly increased in areas of NMDA receptor activation. Radiolabeled CNS 1261 appears to be a good candidate for further development as a single photon emission computed tomography tracer in the investigation of NMDA receptor activation in cerebral ischemia. NUCL MED BIOL 27;6:557-564, 2000. (C) 2000 Elsevier Science Inc. All rights reserved.