Putrescinylthymidine was prepared by the reduction of the Schiff base formed from a 2'-deoxy-5-formyluridine derivative and N-phthaloylputrescine, followed by deprotection. The following deoxyoligonucleotides containing putrescinylthymine (T^P) were synthesized; dodecathymidylic acids containing two to four T^P residues, self-complementary decanucleotides (AAGAATTCTT) and dodecanucleotides (AGATAGCTATCT) in which T residues were partly replaced by T^P, and related oligomers. Oligonucleotides containing T^P were resistant to nuclease S1 digestion and were poor substrates to venom phosphodiesterase. The thermal stability (T_m) of the duplex structure of oligomers containing T^P was not enhanced in spite of the expected electrostatic binding between the putrescinyl and phosphoryl residues, and was rather sequence-dependent.