Rheological investigations have revealed the strong dependency of the solution properties of associative thickener solutions on their shear history. Studies involved the measurement of the apparent solution viscosity both as a function of the shear time at constant shear rates and as a function of shear rate. Thixotropic behavior was observed at all copolymer concentrations studied. Furthermore, the effect of shear history on the viscoelasticity of the solutions was studied with stress relaxation and dynamic measurements. Results are explained in terms of the balance between interchain and intrachain associations of the copolymer molecules in solution, and the kinetics of the molecular reorganization and dissociation-reassociation processes.