The effect of external (magnetic or electric) fields on the swelling of nematic gels dissolved in an isotropic solvent are discussed. We calculate the swelling ratio of the gel, orientational order parameter, and phase diagrams on a temperature-swelling ratio plane under an external field. For a weak external field, we find a first-order volume phase transition between two nematic gels with different orientational ordering. Upon increasing the strength of an external field, a critical point appears on the phase diagram. We also discuss the swelling of nematic gels as a function of the strength of an external field for various temperatures. (C) 2001 American Institute of Physics.