Nanowire and nanotube arrays of transition metal were produced by potentiostatic electrochemical deposition using a polycarbonate membrane filter template (15 to 200 nm in nanopore diameter). Nanotubes are produced by appropriately controlled partial filling of the pore cross section; nanowires result when the filter pore is completely filled. In this manner, nanotubes and nanowires of transition metals (Ni, Co, and Fe) were produced as well as Cu nanowire. The nanowires have an aspect ratio ranging from 50 to 400 dependent on the transient variation of cathodic current. The metal deposition rate must be balanced with that of hydrogen generation (dissolved or evolved as bubbles). The wall thickness of nanotubes, varying from 10 to 70 nm, may be controlled conveniently by adjusting the pH of the solution and the applied electrode potential. Suppression of hydrogen bubble evolution in the nanocapillary structure clearly plays an important role. (c) 2006 The Electrochemical Society.