The dominant source of the effective magnetic field at thenuclei in ferromagnets apparently comes from the Fermi contactcontribution of theselectrons in the core. This contribution wasinvestigated for transition element atoms and ions by means of aseries of spin (or exchange) polarized Hartree-Fock calculations(calculations where one-electron orbitals of differingmsareallowed different radial dependence). Calculations were done forboth free-atom and crude crystalline environments. The sensitivityof the3selectrons to3dbehavior was found to contributeto large differences in calculated effective fields. The differencesbetween core polarizations in metals and ions are discussed withthe use of a model for metallic iron (i.e., a3d8configuration)which is more consistent with neutron diffraction and energyband results than the free atom3d6state (the spin polarized3d8calculation shows an expanded3dcharge density but an almostunchanged spin density relative to the3d6results). The resultant(negative) increase in the effective field is not, however, largeenough to overcome the positive contributions of the outerelectrons (estimated a`, la Marshall) so that although a net negativefield is found it is not large enough in magnitude to agreewith the Mo¨ssbauer measurements. A parallel calculation byGoodings and Heine, based on an artificially expanded3dchargeandspin densities, is also found to be similarly deficient. Theirmodel is discussed and shown to be incompatible with neutronexperiments and energy band calculations.