  In this paper we present some results concerning the effects of two
instantaneous starbursts, separated by a quiescent period, on the dynamical and
chemical evolution of blue compact dwarf galaxies. In particular, we compare
the model results to the galaxy IZw18, which is a very metal-poor, gas-rich
dwarf galaxy, possibly experiencing its first or second burst of star
formation. We follow the evolution of a first weak burst of star formation
followed by a second more intense one occurring after several hundreds million
years. We find that a galactic wind develops only during the second burst and
that metals produced in the burst are preferentially lost relative to the
hydrogen gas. We predict the evolution of several chemical abundances (H, He,
C, N, O, \alpha-elements, Fe) in the gas inside and outside the galaxy, by
taking into account in detail the chemical and energetical contributions from
type II and Ia supernovae. We find that the abundances predicted for the star
forming region are in good agreement with the HII region abundances derived for
IZw18. We also predict the abundances of C, N and O expected for the HI gas to
be compared with future FUSE abundance determinations. We conclude that IZw18
must have experienced two bursts of star formation, one occurred \sim 300 Myr
ago and a present one with an age between 4-7 Myr. However, by taking into
account also other independent estimates, such as the color-magnitude diagram
and the spectral energy distribution of stars in IZw18, and the fact that real
starbursts are not instantaneous, we suggest that it is more likely that the
burst age is between 4 and 15 Myr.
