Helical structures of double-stranded poly[d(A-T)] in solution have been studied by Raman spectroscopy. While the classical right-handed conformation B-type spectra are obtained in the case of sodium chloride solutions, a Z-form Raman spectrum is observed by addition of nickel ions at high sodium concentration, conditions in which the inversion of the circular dichroic spectrum of poly[d(A-T)] is detected, similar to that observed for high-salt poly[d(G-C)] solutions [Bourtayre, P., Liquier, J., Pizzorni, L., & Taillandier, E. (1987) J. Biomol. Struct. Dyn. 5, 97-104]. The characterization of the Z-form spectrum of poly[d(A-T)] is proposed by comparison with previously obtained characteristic Raman lines of Z-form poly[d(G-C)] and poly[d(A-C)].poly[d(G-T)] solutions and of d(CG)3 and d(CGCATGCG) crystals [Thamann, T. J., Lord, R. C., Wang, A. H.-J., & Rich, A. (1981) Nucleic Acids Res. 9, 5443-5457; Benevides, J. M., Wang, A. H.-J., van der Marel, G. A., van Boom, J. H., Rich, A., & Thomas, G. J., Jr. (1984) Nucleic Acids Res. 14, 5913-5925]. Detailed spectroscopic data are presented reflecting the reorientation of the purine-deoxyribose entities (C2'-endo/anti----C3'-endo/syn), the modification of the phosphodiester chain, and the adenosine lines in the 1300-cm-1 region. The role played by the hydrated nickel ions in the B----Z transition is discussed.