value (see Matteucci, 1995). In particular, IZw18 is the galaxy with the lowest metal content known locally, and a N/O ratio roughly solar. Recently, a single starburst chemical evolution model has been successfully developed by Kunth et al. (1995) to reproduce the observed abundances of IZw 18. Kunth et al. (1995) applied Marconi’s et al. (1994) model where star formation is assumed to proceed in short but intense bursts. The contribution to the chemical enrichment of SNe of different type (II, Ia and Ib) is taken into account. The novel feature is the introduction of differential galactic winds where some elements are preferentially lost from the galaxy relative to others. Recently, more and more observational evidence is growing about the existence of such galactic winds in dwarf irregulars as provided by Meurer et al. (1992), Lequeux et al. (1995) and Papaderos et al. (1994). These winds are found to be a crucial ingredient in the model of Kunth et al. (1995) in order to reproduce the high observed N/O ratio in IZw18. To explain the nitrogen abundance of IZw18 this same model assumes also a primary N production from massive stars, as described in the next section.

In this paper 