  The mass assembly of a whole population of sub-Milky Way galaxies is studied
by means of hydrodynamical simulations within the $\Lambda$-CDM cosmology. Our
results show that while dark halos assemble hierarchically, in stellar mass
this trend is inverted in the sense that the smaller the galaxy, the later is
its stellar mass assembly on average. Our star formation and supernovae
feedback implementation in a multi-phase interstellar medium seems to play a
key role on this process. However, the obtained downsizing trend is not yet as
strong as observations show.
