The conformation change of poly(N-isopropylacrylamide) (PNIPAAm) hydrogel as a function of temperature is studied both experimentally and by molecular simulation. The experiments show a Theta-temperature of approximately 305 K and a width of the transition region of approximately 20K. independent of the amount of cross-linker. The conformation change of the hydrogel is caused by a conformation change of the single polymer chains in its backbone. The latter was studied by massively parallel molecular dynamics simulations with GROMACS 4. Different force fields for describing the hydrogel backbone (GROMOS-87, GROMOS-96 53A6 and OPLS-AA) as well as the water molecule (SPC/E and TIP4P) were investigated. In the simulations the mean radius of gyration of the polymer chains was monitored. Two force field combinations (GROMOS-96 53A6 + TIP4P and OPLS-AA + SPC/E) yield the conformation change upon variation of the temperature. The results for OPLS-AA + SPC/E are in fair agreement with the experimental results. (C) 2010 Elsevier B.V. All rights reserved.