We report on the long-term stability of electrical resistivity in Nb-doped TiO2 thin films grown at a high rate by a reactive DC magnetron sputtering from metallic targets. The high deposition rate is obtained by an active control of the oxygen flow during the growth process. Film microstructure and preferential orientation of the crystallites are controlled by the total working pressure in the film growth process. After a heat treatment in vacuum, the film resistivity is in a 10(-3)Omega cm range and the optical transmission higher than 80% in the visible region. While the film is stable when kept under dry nitrogen, significant ageing has been observed when the material is exposed to air. In this case, the DC resistivity steadily increases and fractures form throughout the film. The ageing process is discussed in terms of the evolution of the film microstructure and/or the oxygen exchange through on the film surface. Oxygen uptake from ambient air is confined to a shallow surface region. It is possible that this mechanism triggers the formation/propagation of the fractures that predominantly contribute to the increase in film resistivity.