The problem of achieving robust output regulation for plants described by nonlinear differential equations is discussed. Output regulation means that the closed-loop system is locally exponentially stable about an equilibrium and its output asymptotically tracks any reference signal produced by a fixed external generator. Robust means that the tracking property continues to hold in the occurrence of perturbations in the plant parameters, so long as these perturbations are such that the local stability property is not lost. A nonlinear controller is presented which achieves robust regulation for nonlinear plants; in particular, the paper addresses the case in which the exogenous signals (including the disturbances to be rejected and the references to be tracked) can be measured, i.e. can be exploited by the controller. It is shown that, in this case, the class of nonlinear plants for which robust output regulation can be achieved is much wider with respect to the case in which the exogenous signals cannot be measured.