The resistivities for carbon nanotube networks and highly conducting polymers show similar behaviour, which we ascribe to metallic conduction interrupted by barriers, but the thermopower of the two types of material is very different. The almost linear temperature dependence of the thermopower of highly conducting polymers indicates that the electron-phonon interaction is too small to produce significant superconductivity. For carbon nanotubes, we identify systematic non linearities in the thermopower data and compare them with calculations of thermopower due to sharp peaks in the density of states and to low-temperature enhancement effects. (C) 2003 Elsevier B.V. All rights reserved.