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A unique set of circumstances makes the Rydberg states of atoms ideal for mediating tunable, resonantly enhanced, nonlinear optical interactions. The application of a dc electric field to a vapor of such atoms can induce a second-order or nonlinear susceptibility, which in the absence of the applied field would be forbidden by reasons of symmetry. This paper presents an exact expression, valid within the hydrogenic approximation for field strengths that produce a linear Stark effect, for the resonant contribution to the dc-electric-field-induced susceptibility. Numerical evaluation of this expression for field strengths of the order of 103V/cm predict susceptibilities hundreds of times larger than those of conventional mixing crystals.