We present magnetoresistance measurements aimed at answering several open questions in the understanding of giant magnetoresistance ͑GMR͒. Our measurements are performed on (F1/N/F2/N) multilayers in which N is a nonmagnetic metal ͑Cu or Cr͒, and F1 and F2 are various ferromagnetic metals or alloys. In current perpendicular to the plane ͑CPP͒ measurements on (F1/Cu/Co/Cu) multilayers, where F1 is Fe, Co, or Ni doped with impurities, we observe an inversion of the GMR for V or Cr impurities; this demonstrates, first the importance of the extrinsic effects in GMR and secondly the possibility of obtaining negative as well as positive values of the bulk spin asymmetry coefficient ␤. A compensation thickness with zero GMR is found when the bulk and interface spin asymmetry have opposite signs in the same layer. We interpret the sign of ␤ in models of electronic structure. Measurements on other series of multilayers allow us to show that the interface spin asymmetry coefficient ␥ can also be positive ͑interfaces with Cu͒ or negative ͑interfaces with Cr͒. Finally, the comparison between CPP and CIP data obtained on the same samples sheds light on the different role of the interface intrinsic potential in the two geometries. ͓S0163-1829͑99͒09133-X͔