A number of laboratories are currently using sliding plate rheometers equipped with shear stress transducers to study the nonlinear viscoelasticity and slip of molten polymers. In making such measurements, it is sometimes essential to know to what degree the dynamic response of the shear stress transducer itself is influencing the output signal. Experiments should be designed to minimize this effect, but some attenuation and phase shift is inevitable because of the presence of polymer inside the transducer. We have measured the dynamic response of a shear stress transducer in situ in a sliding plate rheometer for two molten polyethylenes. We have also developed a model for the transducer response. In oscillatory shear experiments above a frequency of 1 Hz, the amplitude ratio and phase lead are significant for both materials studied.