Thermal denaturation of DNA's and the corresponding helix-coil transformation of artificial polyribonucleic and polydeoxyribonucleic acids have been studied extensively both theoretically1-l3 and e ~p e r i m e n t a l l y . l ~-~~ Much less work has been carried out on the properties of these polynucleic acids a t high pressure, and in particular, on the pressure dependence of the helix-coil transition t e m p e r a t ~r e . ~l -~~ Light-scattering techniques have been used in this study to measure the pressure dependence of the helm-coil transition temperature of the two-and three-stranded helices of polyriboadenylic and polyribouridilic acids and of calf thymus DNA. From the slopes of the transition temperature vs. pressure curves and heats of transition obtained from the literature,".34 the following volume changes from these helix-coil transitions have been obtained: (a) -0.96 cc/mole of nucleotide base pairs for the poly (A + U) transition, ( b ) +0.35 cc/ mole of nucleotide base trios for the poly (A + 2U) transition, and ( c ) +2.7 cc/mole of nucleotide base pairs for the DNA transition. The relative magnitudes and signs of these volume changes which show that poly (A + U) is destabilized by increased pressure, whereas poly (A + 2U) and calf thymus DNA are stabilized by increased pressure, indicates that further development of the helix-coil transition theory for polynucleotides is needed.