Conductive fillers are often added to insulating polymers to increase the composite conductivity. Adding fillers often increases the viscosity, which can make the material more difficult to process. In this study, 2-8 wt % multiwalled carbon nanotubes or 2-8 wt % carbon black were added to polycarbonate. The effects on composites' viscosities were studied with small-amplitude oscillatory shear and capillary rheometer testing. The addition of carbon nanotubes and carbon black created yield-stress materials with the yield-stresses increasing with increased filler loadings. The addition of carbon black increased the steady-shear viscosity of the composite at all shear rates for all loadings. The addition of the carbon nanotubes reduced the steady-shear viscosity of the composite at high shear rates for all loadings. This is thought to be due to an internal lubrication of the flow (enhanced disentanglement) due to the presence of the carbon nanotubes. The lubrication effect saturates once 3 wt % carbon nanotubes has been reached. The observed rheological behavior of the carbon nanotube composites is markedly different than usually seen in filled systems. A yield-stress-modified Carreau-Yasuda model was used to interpret the effects of carbon nanotube and carbon black fillers on carbon nanotube/polycarbonate and carbon black/polycarbonate composite viscosity. (C) 2011 Wiley Periodicals, Inc. J Appl Polym Sci 121: 1040-1051, 2011