The dynamic rate of a rubber part must be known to predict adequately the viscoelastic behavior of the rubber part while undergoing dynamic excitation. But accurate and convenient testing systems have not been readily available.The resonant beam system, which is used by most engineering groups, has definite drawbacks. The new dynamic rate system at Cadillac Motor Car Div. is an electro-hydraulic actuated system which excites the rubber part under closed-loop control. The system features a built-in analog computer which accurately determines the spring rate and damping coefficient for selected values of mean load, cyclic amplitude, frequency, and temperature. The need for such a facility, a description of and the procedure in using the resonant beam and the new system, the advantages of the new system as compared to the resonant beam, and the availability of a commercially built electro-hydraulic analog readout system are discussed.