The polymorphism exhibited by the mismatched octamer d(m5C-G-m5C-G-T-G-m5C-G), as a function of the temperature, DNA concentration and ionic strength, was investigated by means of NMR spectroscopy. It is shown that this partly self-complementary DNA fragment, under conditions of low DNA concentration (0.4 mM) and low ionic strength, exclusively prefers to adopt a monomeric hairpin form, which consists of a stem of three Watson-Crick-type base pairs and a loop of only two residues. This in striking contrast with earlier intimations in literature, which postulated that in oligonucleotides loop formations containing only two residues are sterically impossible. Moreover, the hairpin form displays an unusual stability in comparison with previously reported hairpins. A Tm of 332 K and a delta H degree of -130 kJ.mol-1 were calculated for the hairpin to random coil transition. At high DNA concentration (8 mM) and/or upon the addition of sodium chloride the hairpin form occurs in slow exchange with a B-DNA dimer structure (approximately 20% at 270 K, no added salt), which comprises two central GxT-mismatched base pairs with the bases as major tautomers. At higher ionic strength (greater than 100 mM NaCl), or upon the addition of methanol, a third species appears, which is in slow exchange with both the B dimer and the hairpin form. This third species could be identified with a Z DNA form, comprising two GxT mismatches with the bases as major tautomers, with the guanine bases syn and the cytosine and thymine bases anti.