The pressure dependence of the helix-to-coil transition temperature (Tm) of poly[d(A-T)], poly(dA).poly(dT), and poly(dA).poly(dT)2 was studied as a function of sodium chloride concentration. The molar volume change of the transition (delta V) was calculated using the Clapeyron equation and calorimetrically determined enthalpies. The delta V of the transition varied linearly with the logarithm of the salt concentration for both double-stranded polymers. The delta V of poly[d(A-T)] changed from +0.36 to +3.86 cm3 mol-1 as the sodium chloride concentration changed from 0.020 to 1.0 M. For poly(dA).poly-(dT), the delta V varied from +2.60 to +4.59 cm3 mol-1 over the range of 0.020-0.20 M NaCl. No pressure dependence of the van't Hoff enthalpy was observed for the double-helical form of either polymer. The delta V for denaturation of the triple-helical species poly(dA).poly(dT)2 was found to be +7.81 and +10.4 cm3 mol-1 at 1.0 and 3.0 M NaCl, respectively. The observed delta V values are interpreted in terms of changes in the hydration of the polymers arising from release of counterions and changes in the stacking of the bases upon denaturation. Counterion release is assumed to make a net negative contribution to the overall delta V, implying that disruption of the stacking interactions must make a positive volume change to the overall delta V. The difference in the delta V values for the two polymers remains constant as the salt concentration is changed, suggesting a difference in the partial molar volume of their single-stranded forms.(ABSTRACT TRUNCATED AT 250 WORDS)