out to be remarkably deficient relatively to the other elements measured, such as Si, S and Fe as it is possible to see in Table 2.

To circumvent the problem of oxygen uncertainty, Pettini et al. (1995) and Lipman (1995) take Si or S as a proxy for oxygen assuming \[O/Si\] = \[O/S\]=0. This assumption is about true for the halo stars of our own Galaxy (however see later for silicon), but is rather risky for primeval galaxies which might have experienced a different chemical evolution from that of our own Galaxy. In particular, Si and O do not have the same nucleosynthetic origin, silicon may be affected by dust depletion and SII and SiII may be also affected by HII contribution. Also by adopting the Pettini et al. approach for the case of QSO 0000-2169 we have \[N/Si\] = $-0.3$, which stands genuinely high when compared to the other determinations.

What chemical evolution?
========================

Some DLA seem to show similarities to dwarf irregular and blue compact galaxies in terms of abundance pattern. These galaxies, in fact, exhibit relatively high N/O, although with a large spread, at a low overall metallicity Z ranging from 1/10 to 1/30 of the solar 