A solution to the three-dimensional Poisson-Boltzmann equation, generalized to include the finite size of the ions, is presented for the environment of DNA in the B- and Z-conformations. The results clearly indicate that despite the lower linear charge density of the left-handed Z-conformer, there is a higher concentration of Na+ at the immediate surface of the Z-from than at the surface of the B-form. The average concentration of counterions within a 12-Å radius of the DNA is, nonetheless, higher for the B- than for the Z-form. Calculations of the electrostatic interactions of these conformers with an environment of 0.01M monovalent salt show that the salt exerts a greater stabilizing effect on the left-handed conformer than on the right-handed form.