The measured resistance of carbon nanotube networks is often dominated by defects, inter-tube and inter-rope contacts. We show that the peak reported in the frequency-dependent conductivity of single-wall carbon nanotube networks is consistent with metallic conduction interrupted by nonmetallic defects that act as barriers. Such barriers also contribute to the electric field dependence of the conductivity. Using Sheng's model, we calculate the field dependence of fluctuation-assisted tunnelling conduction between metallic regions separated by an insulating barrier. obtaining nonlinearities consistent with our experimental data on carbon nanotube networks. (C) 2002 Elsevier Science B.V. All rights reserved.