Bismuth-based layered compounds have been considered during the last years as interesting materials fur high temperature piezoelectric applications, due to their stability and wide thermal range of the ferroelectric activity. The high electrical conductivity present at high temperatures has been a disadvantage for the potential applications of these compounds. On Bi4Ti3O12 (BIT), different doping such as Nb5+ and W6+ were used to increase resistivity. It has been shown that Nb5+ doping modifies piezoelectric response, passing from relatively high hysteretic to linear and almost completely non-hysteretic. In the present work, the effect of Nb5+ doping related to the conduction response is investigated. Studies of the I-V curves as a function of the temperature allows us, for the first time, to measure the conductivity of these compounds at room temperature (RT) and to discuss which are the microstructural elements that control the conductivity of the material using a serial electrical model. It is shown that at temperatures between RT and 125 degrees C the grain boundary conductivity limits the total conductivity. AL higher temperatures, the material conducts mainly through the bulk of grains. Conduction type, conductivity thermal regimes and chemical composition may explain the change in the piezoelectric behaviour.