  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
IZw18 and NGC1569. Bursts of star formation with short duration are not able to
account for the chemical and morphological properties of these galaxies. The
best way to reproduce the chemical composition of these objects 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 observable in HII regions, since the enriched gas
produced by young stars is in a too hot phase to be detectable with the optical
spectroscopy.
