A parallel plate rheometer has been developed for the study of nonlinear viscoelastic phenomena in molten plastics. By making use of a novel shear stress transducer, the effects of instrument friction and edge phenomena on the stress determination are eliminated. This makes possible the study of the response of a melt to large, transient deformations involving large shear rates. The new rheometer was evaluated by studying the response of a polyisobutylene to interrupted shear at 22°C.