The determination of the dynamic shear modulus and dynamic viscosity of polymer materials on apparatus of the Fitzgerald-Ferry type is discussed. Limiting values of the test piece height for which it is permissible to neglect wave propagation of the oscillations are determined. Agreement of the values of the rheological constants determined with and without allowance for inertia is observed at ratios of test piece height to wavelength not exceeding 0.02–0.05. Above this limit the discrepancy rapidly increases and reaches values excluding the use of the formulas recommended in [1–4]. Relations are presented from which the rheological constants should be determined with allowance for the inertia of the test piece.