We present the first pressure-versus-temperature phase diagram for the helix-to-coil transition of double stranded nucleic acids. The thermodynamic stability of a nucleic acid duplex is a complex function of temperature and pressure and strongly depends on the denaturation temperature, T-M, of the duplex at atmospheric pressure. Depending upon T-M, pressure, and temperature, the phase diagram shows that pressure may stabilize, destabilize, or have no effect on the conformational state of DNA. To verify the phase diagram, we have conducted high-pressure UV melting experiments on poly(dIdC)poly(dIdC), a DNA duplex, poly(rA)poly(rU), an RNA duplex, and poly(dA)poly(rU), a DNA/RNA hybrid duplex. The T-M values of these duplexes have been modulated by altering the solution ionic strength. Significantly, at low salt, these three duplexes have helix-to-coil transition temperatures of 50 degreesC or less. In agreement with the derived phase diagram, we found that the polymeric duplexes were destabilized by pressure if the T-M is < 50 degreesC. However, these duplexes were stabilized by pressure if the T-M is > similar to 50 degreesC. The DNA/RNA hybrid duplex, poly(dA)poly(rU), with a T-M of 31 degreesC in 20 mM NaCl undergoes a pressure-induced helix-to-coil transition at room temperature. This is the first report of pressure-induced denaturation of a nucleic acid duplex and provides new insights into the molecular forces stabilizing these structures.