The thermal denaturation of four oligonucleotides, viz. 3'-d(AT)5pO(CH2)6Opd(AT)5-3' (parAT), 3'-d(AT)5pO(CH2)5Opd(AT)5-5' (antiAT), 3'-d(A)10pO(CH2)6Op(T)10-3' (parA-T) and 3'd(A)10pOX X (CH2)6Opd(T)10-5' (antiA-T) in 0.01 M phosphate buffer at pH 7 in presence 0.1, 0.25, 0.5 and 1.0 M NaCl have been studied. It was shown that at lower temperature (0-20 degrees C) all oligomeres exist as complexes of two (canonic duplex) or four (eight) molecules of oligonucleotides, but at higher temperature (30-70 degrees C)- as hairpins with parallel (parAT and parA-T) of antiparallel (antiAT and antiA-T) orientation of chains. Thermodinamic parameters of separated strands-hairpins and hairpins--"low temperature complexes" transition were computated from the melting curves [A260 (T)] by nonlinear regression. AntiA-T was shown by ethidium bromide binding to exist at low strength (0.01 M phosphate buffer without NaCl) as four-stranded complex from two antiparallel double stranded helices parallely oriented and bonded by satisfy hydrogen-bond of groups not involved in WC-pairing. At higher ionic strength the two of such tetramers was conjugated by hydrophobic interaction into octamers. We speculate that four-stranded complexes serves to bring together, and zipper up two antiparallel double stranded helices at replication of DNA, cross-over of gomologues chromosomes and other biochemically important processes.