AbstractSpiegelmers are high‐affinity l‐enantiomeric oligonucleotide ligands (aptamers) that display high resistance to enzymatic degradation compared to d‐oligonucleotides. Spiegelmers belong to the third generation of aptamers, and are currently extensively investigated as potential therapeutic agents. We have previously developed an original method to label natural oligonucleotides with radiohalogens and particularly with fluorine‐18, the most widely used positron‐emitter, t1/2: 109.8 min. Using the same strategy, we herein report the labelling of Spiegelmers, both with fluorine‐18 for positron emission tomography imaging and iodine‐125 for high resolution autoradiography. Three 25‐mer l‐oligonucleotides have been used, differing (a) by the position of the terminal phosphorothioate monoester group (3′‐ or 5′‐end, and therefore differing by the position of the labelling on the macromolecule) and (b) by the nature of the backbone sugar moiety (2′‐OH or 2′‐H, therefore covering the RNA and DNA series, respectively). N‐(4‐[18F]fluorobenzyl)‐2‐bromoacetamide was synthesized in three radiochemical steps from 4‐cyano‐N,N,N‐trimethylanilinium trifluoromethanesulfonate and HPLC‐purified in 90 min (typical production: 2.2–2.4 GBq starting from a batch of 22–24 GBq of [18F]fluoride). N‐(4‐[125I]iodobenzyl)‐2‐bromoacetamide was synthesized from the corresponding trimethylsilyl derivative (one pot, two radiochemical steps) and HPLC‐purified in 60 min (typical production: 24 MBq starting from 37 MBq of Na[125I]I). Coupling of the Spiegelmers with the appropriate HPLC‐purified [radiolabelled]‐halobenzyl‐2‐bromoacetamide (MeOH/PBS (0.1 M, pH 8), 10 min, 120°C) gave the corresponding labelled conjugated Spiegelmers after RP‐HPLC purification. For fluorine‐18, the whole synthetic procedure yields up to 1.1 GBq of pure labelled Spiegelmers in 160 min with a specific radioactivity of 37–74 GBq/μmol at the end of synthesis starting from 22–24 GBq of [18F]fluoride. For iodine‐125, the whole synthetic procedure allows producing up to 7.4 MBq of pure labelled Spiegelmers in 100 min with a specific radioactivity of 11–37 GBq/μmol starting from 37 MBq of Na[125I]I. Copyright © 2003 John Wiley & Sons, Ltd.