We consider the nonlinear Schrödinger equation on the half-line with a given Dirichlet boundary datum which for large t tends to a periodic function. We assume that this function is sufficiently small, namely that it can be expressed in the form αg b 0 (t), where α is a small constant. Assuming that the Neumann boundary value tends for large t to the periodic function g b 1 (t), we show that g b 1 (t) can be expressed in terms of a perturbation series in α which can be constructed explicitly to any desired order. As an illustration, we compute g b 1 (t) to order α 8 for the particular case that g b 0 (t) is the sum of two exponentials. We also show that there exist particular functions g b 0 (t) for which the above series can be summed up, and therefore, for these functions, g b 1 (t) can be obtained in closed form. The simplest such function is exp(iωt), where ω is a real constant.