The applicability of constitutive parameters for description of electromagnetic processes in heterogeneous media is studied at frequencies at which dimensions of the particles of the media are still substantially smaller than the wavelength in this medium but already are comparable with this parameter. It is proved that consideration for the retardation effects at distances comparable with the characteristic dimensions of inclusion not only changes formulas for constitutive parameters but also enhances the role played by the Drude transition layer in calculation of the reflection coefficient describing reflection of plane waves from a layer of a finite thickness. A modification of the Drude theory is developed that takes into account local anisotropy of the transition layer. It is shown that the developed theory almost eliminates discrepancies between the analytic theory and an exact numerical calculation in a certain frequency region that were observed in previous studies.