The data reported in this study show (1) that the proposed radiolabelling methods does not modify the capcity of the antisense oligonucleotide to hybridize to its target complemetary sequence;(2) that after iv injection of the [18 F] oligonucleotide it is possible to evaluate quantitatively byt PET the kinetics of [18 F] adioactivity in any selected tissue or organ;(3) that these kinetics are highly variable with the nature of the oligonucleotide backbone; and (4) that it is possible to emasure the concentration of [18 F]-labeled metabolites in the plasma during the PET measurements, opening the way to the quantitative evaluation of the tissular concentration of [18 F] oligonucleotide. Hence, the combined techniques reported here constitute the basis to assess directly in vivo the pharmacokinetics of the antisense oligonucleotides, including their metaboloc degradation in the plasma, applicable to virtually any sequence and to some of the most common sugar-phospate backbone modifications. In spite of their promises, clinical applications of oligonucleotides are still to be awaited because a number of difficulties limit their use in vivo. Improvements to be achieved include resistance to plasma and tissular nucleases, better cell membrane penetration (a necessary requisite as the target RNA is intracellular), and reduced toxicity and side effects. Research to circumbent these difficulties follows two directions: Modifications in the phospodiester-deoxyribose backbone and vectorization of antisense oligonucleotides by combining them with various agents, such as liposomes, cationic lipids, nanoparticles, viral vectors, etc. In return however, modification of the chemical nature of the oligonucleotides and vectorization influence the pharmacokinetics behavior of the oligonucleotides. Thus, in order to screen new generations of antisense oligonucleotides proposed for therapeutic goals, it is necessary to acquire easily and rapidly he knowledge of their pharmacokinetics behavior in vivo. The methodology presentes here is relevant to this issue and undoubtedlu demonstrate that it is feasible with [18 F]-labeled ligonucleotides and PET. In addition, the specificity of binding of antisense oligonucleotides to their biological targets could make them food candidates for in vivo imaging of RNAs coding for specifi proteins 21, 22. Further investigations will tell us if the present methodology, which will help to evaluate stratehies for more effective delivery of antisense oligonucleotides to target tissues, alos represents the first step towards nuclei acid imaging with PET.