The resistivity versus temperature relation in bulk ceramic high-temperature superconductors under self-field conditions as well as in weak external magnetic fields is modelled by local Lorentz-force-induced fluxon motion with temperature-dependent pinning. A pinning force density and two viscous drag coefficients in the intergrain and intragrain regions, respectively, can be used as characteristic parameters describing the temperature, current and external field dependences of the sample resistance.