Data have been presented to show that it is possible for rubber compounds when tested in the flexometer to exhibit simultaneously a plastic flow in one direction, and a marked stiffening in another direction, resulting from the elimination of plastic flow by rapid flexing. Both effects are shown to be due mainly to heat generation or temperature effects. The rate of heat generation depends in part on the anisotropy of the rubber, which is much greater than is indicated by some of the ordinary physical tests such as tensile strength, resistance to tearing and to abrasion. The theory that the degree of anisotropy is influenced by the shape and orientation of the pigment particles and of the anisotropy of the rubber itself has been in a measure experimentally verified.