  Blue Compact Dwarf and Dwarf Irregular galaxies are generally believed to be
unevolved objects, due to their blue colors, compact appearance and large gas
fractions. Many of these objects show an ongoing intense burst of star
formation or have experienced it in the recent past. By means of 2-D
hydrodynamical simulations, coupled with detailed chemical yields originating
from SNeII, SNeIa, and intermediate-mass stars, we study the dynamical and
chemical evolution of model galaxies with structural parameters similar to
NGC1569, a prototypical starburst galaxy. A burst of star formation with short
duration is not able to account for the chemical and morphological properties
of this galaxy. The best way to reproduce the chemical composition of this
object is by assuming long-lasting episodes of star formation and a more recent
burst, separated from the previous episodes by a short quiescent period. The
last burst of star formation, in most of the explored cases, does not affect
the chemical composition of the galaxy, since the enriched gas produced by
young stars is in a too hot phase to be detectable with the optical
spectroscopy. Models assuming the infall of a big cloud towards the center of
the galaxy reproduce the chemical composition of the NGC1569, but the pressure
exercised by the cloud hampers the expansion of the galactic wind, at variance
with what observed in NGC1569.
