  The isoscalar giant dipole resonance structure in $^{208}$Pb is calculated in
the framework of a fully consistent relativistic random phase approximation,
based on effective mean-field Lagrangians with nonlinear meson self-interaction
terms. The results are compared with recent experimental data and with
calculations performed in the Hartree-Fock plus RPA framework. Two basic
isoscalar dipole modes are identified from the analysis of the velocity
distributions. The discrepancy between the calculated strength distributions
and current experimental data is discussed, as well as the implications for the
determination of the nuclear matter incompressibility.
