We present a molecular dynamics simulation method for adsorption/desorption in a sample of a model porous material from a fluid reservoir at a fixed chemical potential and temperature. The method was developed to investigate the stability of the states associated with hysteresis in the adsorption/desorption isotherms obtained in grand canonical Monte Carlo (GCMC) simulations. For a model of a simple fluid adsorbed in a silica gel, we find that the hysteresis loops obtained in the molecular dynamics calculations are essentially identical to those obtained in GCMC simulations of the same system. The significance of this result is discussed.