The unambiguous assignment of the aromatic protons of the duplex d(CA)3.d(TG)3 was carried out with the aid of HMQC and HMBC inverse detected C-13 NMR experiments at T = 278 K. The NOE connections of these protons with the imino protons of the complementary bases - established by 2D NOESY experiments - indicated that the duplex is characterized by a canonical Watson-Crick base-pairing pattern. Monodimensional H-1 NMR experiments, also carried out at T = 278 K, in the presence of varying amounts of NaCl and with water suppression by presaturation and by spin echo techniques, showed that the imino protons of the TA and CG pairs of the duplex exchange with water through a process of local base pair opening and with different exchange rates.