  We performed an optical spectroscopic monitoring of the blazar 3C 454.3 from
September 2003 to July 2008. Sixteen optical spectra were obtained during
different runs, which constitute the first spectroscopic monitoring done in the
rest-frame UV region (z=0.859). An overall flux variation of the MgII (2800 A)
by a factor ~ 3 was observed, while the corresponding UV continuum (F_cont at
3000 A) changed by a factor ~ 14. The MgII emission lines respond
proportionally to the continuum variations when the source is in a low-activity
state. In contrast, near the optical outbursts detected in 2005 and 2007, the
MgII emission lines showed little response to the continuum flux variations.
During the monitored period the UV FeII flux changed by a factor ~ 6 and
correlated with F_cont (r = 0.92). A negative correlation between EW(Mg II) and
F_cont was found, i.e. the so-called "Intrinsic Baldwin Effect".
