A theoretical study of indices potentially useful for investigation of the reactivity of the recently discovered Z-DNA double helix is presented. The electrostatic potential minima and the steric accessibility of reactive sites are calculated. The effect of screening the phosphate groups by metal cations is investigated. The results are compared with those for the B-DNA double helix.A theoretical study of indices potentially useful for investigation of the reactivity of the recently discovered Z-DNA double helix is presented. The electrostatic potential minima and the steric accessibility of reactive sites are calculated. The effect of screening the phosphate groups by metal cations is investigated. The results are compared with those for the B-DNA double helix.