An apparatus for carrying out torsional shear tests of polymers under hydrostatic pressure environment at variable rotational speeds has been developed and tested. The apparatus is capable of containing pressures up to 7 kilobars and producing rotational speeds to 1000 rpm. A number of important and unique features have been incorporated into the design of the apparatus. The apparatus consists of two interconnected thick-walled cylinders, one for testing and the other for pressure compensation. The rotation rate is automatically maintained at a constant level throughout a test by a feedback amplifier circuit. The magnitude of the applied torque is detected by a load cell mounted within the torque shaft, unexposed to the pressure medium and unaffected by the friction in the seals. The angular displacement is sensed both internally and externally, respectively, by an electrically activated contact point moving over a toothed wheel mounted on the rotating end of the specimen and by a rotary variable differential transformer (RVDT). The signals from the torsion cell, the rotary contact point, and the RVDT are monitored and recorded by an integrated electronic system. The shear stress–strain diagrams of a polymer, polypropylene (PP), have been obtained at various pressures. The data show the dependence of elastic and plastic behaviors of PP on hydrostatic pressure.