The concentration dependence of mechanical properties of polymer solutions may be attributed to the changes in relaxation spectra due to the hydrodynamic and thermodynamic interactions between molecules in solution. In this work, the changes in intramolecular modes of motion caused by intermolecular interactions are calculated on the random-flight model of polymer chains by using the perturbation method developed by Fixman and the representation of the reciprocal intersegmental distances devised by Imai. The Huggins' constant in the viscosity equation is shown to be a universal constant, 0.842, in theta solvents independent of molecular weight and to be a decreasing function of solvent power. Its behavior in the neighborhood of the theta state can be written as KH = 0.842 − 0.498(α2 − 1), where α is the expansion factor for the intrinsic viscosity. These results are compared with recent experiments. The concentration dependence of dynamic shear moduli is also calculated on the same model. The results can describe, at least qualitatively, the apparent transition of frequency dependence of dynamic shear moduli from Zimm type to Rouse type which occurs with the increase in concentration, observed by Ferry et al., Lamb et al., and others. This transition comes from the superposition of the intramolecular contribution (Zimm type) and the intermolecular contribution.