Cyclic oligodeoxyribonucleotides have been synthesized in satisfactory yields by an easy procedure using polyethylene glycol (PEG) as soluble supporting polymer. The present method is particularly suitable for a medium to large scale synthesis.Early studies involving biological properties of synthetic1 and natural2 cyclic oligonucleotides stimulated a great deal of interest in such kind of molecules.The recent findings3 that circular single-stranded oligodeoxyribonucleotides having two runs of pyrimidines can bind tightly and sequence selectively to single-stranded polypurine DNA and RNA by forming triple helical complexes open up new perspectives to the design of more efficient DNA-and RNA-binding molecules.Our attention is being focused on the synthesis of medium-sized rings, since none of the synthetic procedures described so far proved to be really suitable for the preparation of such molecules on a relatively large scale. Following „classical“ solution methods, which in principle should easily allow syntheses