Addition of small amounts of maleic anhydride containing copolymers remarkably increases the melt viscosity of the m x D,6 polyamides (PA). Random copolymers of styrene and maleic anhydride, P(S-co-MA), have been melt blended with PA in a ratio ranging from 0.5 to 5 wt%. A linear dependence has been observed for the blend viscosity on the molar ratio of the maleic anhydride subunits in the copolymer and the amino end-groups of PA. This linear relationship holds even for blends containing a molar excess of anhydride compared to amine. These rheological effects are of prime importance for the highly desirable control of the PA processing.