  We present the analysis of single epoch long slit spectra of the three
brightest images of the gravitationally lensed system J1131-1231. These spectra
provide one of the clearest observational evidence for differential
micro-lensing of broad emission lines (BELs) in a gravitationally lensed
quasar. The micro-lensing effect enables us: (1) to confirm that the width of
the emission lines is anti-correlated to the size of the emitting region; (2)
to show that the bulk of FeII is emitted in the outer parts of the Broad Line
Region (BLR) while another fraction of FeII is produced in a compact region;
(3) to derive interesting informations on the origin of the narrow intrinsic
MgII absorption doublet observed in that system.
