  The role of the meson-exchange current correction to the nuclear charge
operator is studied in electron scattering processes involving the excitation
of medium and heavy nuclei to energies up to the quasi-elastic peak. The effect
of these contributions in the quasi-free electron scattering process is a
reduction of at most a 3% in the longitudinal response at the energy of the
peak, a value which is below the experimental error and must not be taken into
account in calculations in this energy region. On the other hand, the
excitation of low-lying nuclear levels of neutronic character shows, with
respect to the protonic ones, a considerable effect due to the inclusion of the
two-body term in the charge operator. More realistic calculations, such as
those performed in the random-phase approximation framework, give rise to a
mixing of one particle-one hole configurations of both kinds which reduce these
effects. However, it has been found that the excitation of some of these levels
is sizeably affected by the meson-exchange contribution. More precise
experimental data concerning some of these states, such as e.g. the high-spin
states in 208Pb, could throw some light in the problem of a more feasible
determination of these effects and, as a consequence, could provide an
alternative procedure to obtain the charge neutron form factor.
