a reasonable quantity of N during a short burst (no longer than 20 Myr), as discussed in Kunth et al. (1995), is to require that massive stars produce a substantial amount of primary nitrogen. This claim was already made by Matteucci (1986) in order to explain the \[N/O\] abundances in the solar neighbourhood. Recently, calculations by Woosley and Weaver (private communication) seem to support the possibility that N is produced by C and O synthesized inside massive metal poor stars as a primary element. Therefore, as in Kunth et al. (1995) we have taken this possibility into account in the present calculation.

Theoretical prescriptions for the evolution of the solar vicinity
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The model adopted for the evolution of the solar neighbourhood is the same as in Matteucci and François (1992) and the basic equations are similar to Eq (1), the only difference being the absence of a galactic wind, namely all the $w_{i}=0$. The main differences between the model for dwarf irregulars and the model for the solar vicinity is that in the latter case the star formation rate is continuous and the IMF for massive stars is steeper than a Salpeter IMF (i.e. Scalo 1986). The time scale 