The structures of B and Z forms of poly(dG-dC).poly(dG-dC) in oriented hydrated films have been investigated by infrared spectroscopy. First, the infrared linear dichroism spectrum of poly(dG-dC).poly(dG-dC) in B form (high relative humidity and 2 M Na+) was recorded. The spectrum of the Z form was recorded at a lower relative humidity and higher sodium content (4 M). The experimental conditions were similar to those used in x-ray diffraction studies of left-handed poly(dG-dC).poly(dG-dC) fibers. A qualitative agreement was found between the calculated values of the angles characteristic of the orientation of PO-2 groups with respect to the helical axis in the ZI form and the experimental values. The infrared spectra of B and Z form and the experimental values. The infrared spectra of B and Z forms show some significant differences in H2O and 2H2O. The deuteration rates of exchangeable protons involved in hydrogen bonds between guanine and cytosine residues were deduced from the changes in absorbance near 1700 cm-1. In the B form, the exchange half-time is of the order of 20 min. In the Z form, the exchange half-time of some protons is very slow, of the order of 24 hr. Because of the similarity between these values and those previously reported for the high-salt and low-salt form of poly(dG-dC).poly(dG-dC) in solution, we conclude that the high-salt form of poly(dG-dC).poly(dG-dC) in solution belongs to the Z family.