Metallicity in damped Lyman-alpha systems: evolution or bias?

Assuming that damped Lyman-alpha (DLA) systems are galactic discs, we calcu late the corresponding evolution of metal abundances. We use detailed multi zone models of galactic chemical evolution (reproducing successfully the ob served properties of disc galaxies) and appropriate statistics (including g eometrical probability factors) to calculate the average metallicity as a f unction of redshift. The results are compatible with available observations , provided that observational biases are taken into account, as suggested b y Boisse et al. In particular, high column density and high-metallicity sys tems are not detected because the light of backround quasars is severely ex tinguished, while low column density and low-metallicity systems are not de tectable through their absorption lines by current surveys. We show that th ese observational constraints lead to a 'no-evolution' picture for the DLA metallicity, which does not allow us to draw strong conclusions about the n ature of those systems or about their role in 'cosmic chemical evolution'.