Variations in viscosity, extrudate swell, and end correction were determined for natural Guayule rubber, natural Hevea rubber, and synthetic polyisoprene as a function of shear rate(10–104 s−1),temperature (120, 140, 160 °C), mastication time in a two‐roll mill (0, 5, 15 min), and carbon black load (30, 50, 70 phr). Viscosity decreases with shear rate, temperature, and mastication time and increases with carbon black load. Extrudate swell increases with shear rate and decreases with temperature, mastication time, and carbon black loading. End correction (for gums) increases with shear rate and decreases with temperature and mastication time. End correction (for blacks) increases with shear rate but the effect of temperature and carbon black loading are interrelated, as the effect of one depends on the magnitude of the other. Also, at constant shear rate and temperature, end correction for blacks is lower than for gums. Finally, viscosities for natural Guayule rubber are similar in magnitude to those of natural Hevea rubber and synthetic polyisoprene, whereas extrudate swell and end correction are somewhat higher for Guayule rubber than for Hevea and synthetic polyisoprene rubbers.