Fractions of polystyrene, varying in molecular weight from 0.3×106to 10×106approximately, were prepared by careful tertiary fractionation. Inherent viscosities were determined at various rates of shear in the approximate range 100–20 000 sec−1, viscosity measurements being made at several different temperatures in benzene and toluene (good solvents for polystyrene), in butanone (an intermediate solvent), and in cyclohexane (a poor solvent). From these values intrinsic viscosities at various rates of shear were determined. These viscosity functions are sensitive to shear rate at high molecular weights; at a given temperature the shear effects are greater the higher the molecular weight and the better the solvent, and with increasing temperature they decrease in a good solvent and increase in a poor one This behavior is shown to be consistent with current ideas regarding the size and shape of polymer molecules in solution as affected by weight, temperature, and solvent power. It is concluded, among other things, that with flexible polymers (or polymer fractions) of very high molecular weight, measurements of intrinsic viscosity should be made in a poor solvent at a low temperature.