Concentrated solutions of nearly monodisperse polystyrene samples in cyclohexane in the theta condition and in the entanglement-free region have been studied by means of the depolarized photon-correlation spectroscopy and viscosity measurements. It is shown that the viscoelastic behavior of the studied systems is described by the Rouse theory and that in agreement with the theoretical analysis the main polymer dynamic process probed by the depolarized Rayleigh scattering is basically the reorientational motion associated with a Rouse segment of the polymer chain, whose relaxation is independent of the scattering angle and the molecular weight. In addition to the main dynamic process, the tail region of a very fast process associated with the sub-Rouse-segmental motions can be observed, whose existence is expected from the theoretical analysis.