  Using a sample of 57 VLT FORS spectra in the redshift range 1.37<z<3.40
(selected mainly from the FORS Deep Field survey) and a comparison sample with
36 IUE spectra of local (z ~ 0) starburst galaxies we derive CIV and SiIV
equivalent width values and estimate metallicities of starburst galaxies as a
function of redshift. Assuming that a calibration of the CIV equivalent widths
in terms of the metallicity based on the local sample of starburst galaxies is
applicable to high-z objects, we find a significant increase of the average
metallicities from about 0.16 Z_sun at the cosmic epoch corresponding to z ~
3.2 to about 0.42 Z_sun at z ~ 2.3. A significant further increase in
metallicity during later epochs cannot be detected in our data. Compared to the
local starburst galaxies our high-redshift objects tend to be overluminous for
a fixed metallicity. Our observational results are in good agreement with
published observational data by other authors and with theoretical predictions
of the cosmic chemical evolution.
