We apply a space-dependent external potential to the fluid such as to produce both a high density, virtually homogeneous, region and also a low density, not necessarily homogeneous, region. We easily measure the chemical potential in the low density region by Widom’s fictitious-particle-insertion method by computer simulation. By constancy of the chemical potential the value is the same in the high density region, which is difficult to measure directly by this or any other method. The method has been tested for Lennard-Jones fluids, for which independent values of chemical potential at high density are known. The method is quite general and can be used for molecular liquids, polymer melts, mixtures, etc.