Several medium and high molecular weight polar fluids, when subjected to shearing forces above a certain rate of shear, showed a rheopectic behavior. The observation was made on some polyesters, an epoxy compound, and on concentrated solutions of these materials in nonpolar solvents. The experiments were carried out in a rotating type viscometer with cone-and-plate arrangements. The following explanation for the observed rheopexy is proposed: above a critical rate of shear, the polar macromolecules after alignment in the field of shear are forming quasicrystallites and a rheological network structure due to intermolecular forces. Because of the simultaneously occurring process of the breaking of structures in the field of shear, a state of pseudoequilibrium is reached. It was found that when the experiments were interrupted at the stage of such a pseudo-equilibrium for a short time, a thixotropic strengthening of the existing structure occurred; under renewed shear the stress decreased to its value before the interruption. After interruption of shear for longer periods (hours, days), a breakdown of the rheopectic structure was observed.