Andrea Sacchetti Nonlinear time-dependent Schrodinger equations: the Gross-Pitaevskii equation with double-well potential. (4641K, Postscript) ABSTRACT. We consider a class of Schrodinger equations with a symmetric double-well potential and an external, both repulsive and attractive, nonlinear perturbation. We show that, under certain conditions, the reduction of the time-dependent equation to a two-mode equation gives the dominant term of the solution with a precise estimate of the error.