A high-pressure sliding plate rheometer has been developed to investigate the effect of pressure on the rheological behavior of molten polymers and elastomers. The new rheometer operates at pressures up to 70 MPa and temperatures up to 225 degrees C. The sample is subjected to simple shear, and the resulting shear stress is measured locally using a shear stress transducer. This design eliminates the inhomogeneities in pressure and shear rate that occur in high pressure capillary and slit rheometers. Preliminary evaluation of the new instrument was carried out using a linear low density polyethylene. Viscosity curves were generated at pressures ranging from atmospheric pressure to 70 MPa, and the pressure coefficient of viscosity was determined. Experiments were also carried out in step strain and large amplitude oscillatory shear, demonstrating the new rheometer＇s use to study the nonlinear viscoelastic behavior of molten polymers. Finally, this instrument was used to study strain-induced crystallization.