Thymidine was converted into 5-formyl-2'-deoxyuridine (1), which was incorporated into oligonucleotides, 5'd(GGAGA1CTCC)3' (I-1) and 5'd(GCTGC1GCGAAAGCTG)3' (II-1). To avoid side-reactions and degradation, protection of the formyl group of 1 using a newly developed protecting group, N, N-di-(3, 5-dichlorophenyl)ethylenediamine, was necessary. Compound 1 was unstable under the conditions employed for enzymatic complete degestion of oligonucleotides, so that a peak corresponding to 1 was notodetected clearly by HPLC analysis of a nucleoside mixture obtained by complete hydrolysis of I-1. Therefore, the oligonucleotide I-1 was treated with cyanomethylenetriphenylphosphorane to give an oligonucleotide containing (E) and (Z)-5-(2-cyanovinyl)-2'-deoxyuridine, which was then hydrolyzed, and the newly generated nucleosides were detected by HPLC analysis. The T_m of the self-complementary oligonucleotide I-1 (40°C) was higher than that of the parent oligonucleotide, 5'd(GGAGATCTCC)3', (31°C) in a buffer containing 0.01M sodium phosphate (pH 7.0) and 0.1M NaCl. DNA replication study on a template-primer system [primer, 5'd(³²P-CAGCTTTCGC)3'; template, 3'd(GTCGAAAGCGXCGTCG)5' (X=1 or T)] showed that dATP was incorporated into the DNA strand at a site opposite to 1 by Klenow DNA polymerase, but with a reduced rate. The formyl group of 1 in the oligonucleotides reacted with amines to geve Schiff base derivatives.