  We consider an 8--dimensional gravitational theory, which possesses a
principle fiber bundle structure, with Lorentz--scalar fields coupled to the
metric. One of them plays the role of a Higgs field and the other one that of a
dilaton field. The effective cosmological constant is interpreted as a Higgs
potential. The Yukawa couplings are introduce by hand. The extra dimensions
constitute a $SU(2)_{L} \times U(1)_{Y} \times SU(2)_{R}$ group manifold. Dirac
fields are coupled to the potentials derived from the metric. As result, we
obtain an effective four--dimensional theory which contains all couplings of a
Weinberg--Salam--Glashow theory in a curved space-time. The masses of the gauge
bosons and of the first two fermion families are given by the theory.
