The rheological properties of aqueous polyurethane dispersions are reported from the dilute regime up to mass concentrations in excess of 40 wt.%. Particle size decreases and the particles are more resistant to shear induced deformation with increasing ionic strength and, at very low ionic strength, the reduced viscosity passes through a minimum with increasing concentration. As the mass fraction of polyurethane increases, the dispersions shear thin. The zero shear rate viscosity diverges with the development of an apparent yield stress at the highest concentrations probed. Stress sweep experiments show that deviation from linear viscoelasticity occurs at strains of less than 0.05% for the moderately concentrated dispersions. The Cox-Merz rule is obeyed by these dispersions only at strains in the linear viscoelastic region and at concentrations below the gel point. As the ionic strength is increased at fixed particle concentration, viscosities pass through a minimum. These phenomena are discussed in terms of particle deformability and an interplay of electrostatic and steric repulsive forces. (C)1999 Elsevier Science B.V. All rights reserved.