The pressure dependence of the helix-coil transition temperature (Tm) of poly[d(G-C)] was studied as a function of sodium ion concentration in phosphate buffer. 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 increased from +4.80 (+/- 0.56) to +6.03 (+/- 0.76) mL mol-1 as the sodium ion concentration changed from 0.052 to 1.0M. The van't Hoff enthalpy of the transition calculated from the half-width of the differentiated transition displayed negligible pressure dependence; however, the value of this parameter decreased with increasing sodium ion concentration, indicating a decrease in the size of the cooperative unit. The volume change of the transition exhibits the largest magnitude of any double-stranded DNA polymer measured using this technique. For poly[d(G-C)] the magnitude of the change in delta V with sodium ion concentration (0.94 +/- 0.05 mL mol-1) is approximately one-half that observed for either poly[d(A-T)] or poly(dA).poly(dT). The delta V values are interpreted as arising from changes in the hydration of the polymer due to the release of counterions and changes in the stacking of the bases of the coil form. As a consequence of solvent electrostriction, the release of counterions makes a net negative contribution to the total delta V, implying that disruption of the stacking interactions contributes a positive volume change to the total delta V.(ABSTRACT TRUNCATED AT 250 WORDS)