N-(4-Methoxybenzyl)-N'-(5-nitro-1,3-thiazol-2-yl)urea (AR-A014418), a highly selective inhibitor of glycogen synthase kinase-3beta (GSK-3beta), was radiolabelled with carbon-11 (half-life=20.4min) for cerebral positron emission tomography (PET) studies. Reaction of desmethyl AR-A014418 with [(11)C]CH(3)I produced [(11)C]AR-A014418 in 17% decay-corrected radiochemical yield, based on [(11)C]CO(2), with 3230mCi/micromol specific activity after a 30min synthesis time. The desmethyl precursor of AR-A014418 was synthesized in 23% yield by a novel one-pot reaction of 2-amino-5-nitrothiazole with in situ generated TMS-protected 4-hydroxybenzylisocyanate, following deprotection with acid. Ex vivo biodistribution studies were conducted after [(11)C]AR-A014418 was administered via tail vein injection into Sprague-Dawley rats. Very low levels of radioactivity were found in all brain regions (0.08% injected dose/gram of tissue) at 5 and 30min post-injection, uncorrected for vascular compartment. Considering the extremely poor brain penetration of [(11)C]AR-A014418 this compound cannot be used to study GSK-3beta in cerebral PET studies. Furthermore, the specific pharmacological mechanism(s) of antidepressant-like activity attributed to AR-A014418 should be investigated.