Ethylene-propylene copolymer, a typical stereo rubber, has been investigated by capillary viscometry. Ethylene-propylene copolymer possesses high thermooxidative stability, which has made it possible to study its viscosity properties, determine the onset of elastic turbulence and boundary slip, and measure the slip rate over a very broad temperature interval, from room temperature to 260° C. The flow of elastomers differs from that of thermoplastics in that at relatively low strain rates flow is complicated by the boundary slip effect. The mean boundary slip velocities of the copolymer at shear stresses above 106 dynes/cm2 are measured in tens of centimeters per second. As the temperature rises, they rapidly increase.