Using c-axis La2CuO4 thin films, we first demonstrate that the electrochemical oxidation mainly occurs along the c axis. Then we induce oxygen-rich regions electrochemically into an otherwise oxygen-deficient matrix. On a microscopic scale the extra oxygen introduced creates metallic and/or superconducting regions in the insulating matrix. Contrary to other lithographic techniques in which large amounts of material are either removed or deposited, this technique does not induce significant height differences. (C) 1996 American Institute of Physics.