Dynamics of the nonlinear Schrödinger equation in the presence of a constant electricfield is studied. Both discrete and continuous limits of the model are considered. For thediscrete limit, a probabilistic description of subdiffusion is suggested and asubdiffusive spreading of a wave packet is explained in the framework of a continuous timerandom walk. In the continuous limit, the biased nonlinear Schrödinger equation is shownto be integrable, and solutions in the form of the Painlevé transcendents are obtained.
