In this paper, the theological properties of solutions of high molecular weight partially hydrolysed polyacrylamide in mixtures of glycerol and water are presented. The influence of the amount of glycerol and of salt on the conformation of the macromolecules is studied through the measurement of the reduced viscosity. When shear stress or shear rate is applied to the sample, the viscosity depends on time and displays a transient regime with strong instabilities followed by a steady state. Then, the non-Newtonian viscosity appears as a superposition of two effects: a sharp increase of the viscosity, which is an effect of time, and a slight shear thickening. All these phenomena depend on temperature and on polymer and salt concentrations. They can be interpreted as a consequence of the formation and destruction of aggregates, depending on the balance between intra- and inter-molecular interactions.