Gold surfaces modified with thiol-derivatized DNA duplexes have been investigated as a function of applied electrochemical potential via atomic force microscopy(EC-AFM). At open circuit, monolayers of well-packed DNA helices form with a film depth of 45(3) Angstrom. On the basis of the anisotropic dimensions of these 15 base pair duplexes (20 Angstrom in diameter versus 50 Angstrom in length), this corresponds to an average similar to 45 degrees orientation of the helical axis with respect to the gold surface. Under potential control, the monolayer thickness (and therefore the orientation of the helices) changes dramatically with applied potential. At potentials negative of similar to 0.45 V (versus a Ag wire quasi-reference electrode) film thicknesses of similar to 55 Angstrom are observed, whereas at more positive potentials the monolayer thickness drops to a limiting value of similar to 20 Angstrom. These results are consistent with a morphology change in which the helices either stand straight up or lie nat down on the metal surface, depending on the electrode-potential relative to the potential of zero charge (pzc). This voltage-induced morphology change is reversible and effectively constitutes a nanoscale mechanical "switch".