Optimal conditions for the developing stage of fixed gratings in LiNbO 3 have been theoretically determined. The analysis, that includes an external applied field is based on the well known band transport model. In particular, the role of the photovoltaic effect on the developing process has been studied in detail. Moreover, the applied field as well as an effective photovoltaic field are shown to be key parameters in order to optimize the developing efficiencies. The influence of other parameters such as grating wave vector, doping level and reduction state of the sample has also been analyzed.