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 ([¹¹C]1). Initial radioactivity in the brain after intravenous injection of [¹¹C]1 was a 2-fold that of [¹¹C]L-703, 717 in mice. Rapid bioconversion of [¹¹C]1 into [¹¹C]L-703, 717 was demonstrated by metabolite analyses of rat brain after [¹¹C]1 injection. Ex vivo autoradiography of [¹¹C]1 in rat brain showed the same cerebellar localization of radioactivity as [¹¹C]L-703, 717. These results indicate that 1 is a promising pharmacological tool as a prodrug of L-703, 717 with improved BBB permeability.