We investigate the rheological behavior of semidilute salt-free solutions of a high molecular weight poly(acrylamide-co-sodium acrylate). The frequency dependence of the complex shear modulus G*(omega) was measured in a large concentration range. Results give support to a recent model based on the scaling theory. It is found in particular that the zero-shear rate viscosity varies as C-3/2 in the entangled semidilute regime over 3 decades of concentration.