The dielectric response to photoexcitation of the Green Fluorescent Protein (GFP) chromophore and contributions to it from water and GFP are computed by molecular simulations using a force field for the chromophore parametrized by ab initio calculations of ground and excited states. The chromophore is embedded in the beta-barrel where it is surrounded by about 10 waters, which are found to play a significant role in slow dielectric relaxation. Dynamics of hydrogen bonds between water and GFP is examined and found to be slower and more heterogeneous for hydrogen bonds inside the beta-barrel than in the hydration layer around GFP. (C) 2013 Elsevier B. V. All rights reserved.