4-Acetoxy derivative (1) of L-703,717, a high-affinity (IC50=4.5 nM) antagonist for the glycine site of NMDA receptors, was synthesized and its brain uptake was examined using a carbon-11 labeled analog ([C-11]1). Initial radioactivity in the brain after intravenous injection of [C-11]1 was a 2-fold that of [C-11]L-703,717 in mice. Rapid bioconversion of [C-11]1 into [C-11]L-703,717 was demonstrated by metabolite analyses of rat brain after [C-11]1 injection. Ex vivo autoradiography of [C-11]1 in rat brain showed the same cerebellar localization of radioactivity as [C-11]L-703,717. These results indicate that 1 is a promising pharmacological tool as a prodrug of L-703,717 with improved BBB permeability.