  In this work we present the predictions of the ``two-infall model''
concerning the evolution of D, 3He and 4He in the solar vicinity, as well as
their distribution along the Galactic disk. Our results show that, when
adopting detailed yields taking into account the extra-mixing process in low
and intermediate mass stars, the problem of the overproduction of 3He by the
chemical evolution models is solved. The predicted distribution of 3He along
the disk is also in agreement with the observations. We also predict the
distributions of D/H, D/O and D/N along the disk, in particular D abundances
close to the primordial value are predicted in the outer regions of the Galaxy.
The predicted D/H, D/O and D/N abundances in the local interstellar medium are
in agreement with the mean values observed by the Far Ultraviolet Spectroscopic
Explorer mission, although a large spread in the D abundance is present in the
data. Finally, by means of our chemical evolution model, we can constrain the
primordial value of the deuterium abundance, and we find a value of (D/H)_p < 4
10(-5) which implies Omega_b h^2 > 0.017, in agreement with the values from the
Cosmic Microwave Background radiation analysis. This value in turn implies a
primordial 4He abundance Y_p > 0.244.
