  We clarify the mass dependence of the effective prepotential in N=2
supersymmetric SU(N_c) gauge theories with an arbitrary number N_f<2N_c of
flavors. The resulting differential equation for the prepotential extends the
equations obtained previously for SU(2) and for zero masses. It can be viewed
as an exact renormalization group equation for the prepotential, with the beta
function given by a modular form. We derive an explicit formula for this
modular form when N_f=0, and verify the equation to 2-instanton order in the
weak-coupling regime for arbitrary N_f and N_c.
