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We analyzed the evolution of the metallicity of the gas with the redshift fora sample of AGNs in a very wide redshift range (0<z<4) using ultravioletemission-lines from the narrow-line regions (NLRs) and photoionization models.The new index C43=log(CIV+CIII])/HeII is suggested as a metallicity indicatorfor AGNs. Based on this indicator, we confirmed the no metallicity evolution ofNLRs with the redshift pointed out by previous works. We found that metallicityof AGNs shows similar evolution than the one predicted by cosmic semi-analyticmodels of galaxy formation set within the Cold Dark Matter merging hierarchy(for z < 3). Our results predict a mean metallicity for local objects inagreement with the solar value (12+log(O/H)=8.69). This value is about the samethat the maximum oxygen abundance value derived for the central parts of localspiral galaxies. Very low metallicity log(Z/Z_{\odot})~ -0.8 for some objectsin the range 1.5 < z <3 is derived.

Metallicity evolution of AGNs from UV emission-lines based on a new index