Circular dichroism (CD) spectra in the vacuum UV region for different conformations of poly d(G-C).poly d(G-C) and poly r(G-C)·poly r(G-C) are very characteristic. The CD of the RNA in the A-form (6 N NaClO4 and 22°C) is very similar to that of the DNA in 80% alcohol where it is believed to be in the A-form. With the exception of the longest wavelength transition, the CD of the RNA in 6 N NaClO4 at 46°C is similar.ar to the CD of the DNA under conditions where it is believed to be in the Z-form (2 M NaClO4 This substantiates that poly r(G-C)·poly r(G-C) assumes a left-handed Z-conformation in 6 M NaClO4 above 35°C. CD spectra for the left-handed Z-forms of both the RNA and DNA are characterized by an intense negative peak at 190-195 nm, a crossover at about 184 nm, and an intense positive peak below 180 nm. The right-handed A- and B-forms of RNA and DNA all have an intense positive peak in their CD spectra near 186 nm. The large difference in CD in the range 185-195 rim for right- and left-handed conformations of nucleic acids can be used to identify the sense of helix winding.