Dissipative mechanisms in Bi1.8Pb0.4Sr2Ca2Cu3Ox tapes were investigated using magnetoresistance measurements and V-I characteristics. A considerable broadening of the resistive transition in applied magnetic field was observed up to a field of 0.5 T. The temperature and magnetic field dependence of the resistance R(T,B) were fitted to the Arrhenius relation and from which the magnetic field dependence of the pinning energy U-0(B) similar to B-alpha with alpha approximate to 0.9 was derived. The variations of the critical current densities J(c) with temperature and magnetic field before and after gamma-irradiation were also studied. The temperature dependence of J(c)(T) was fitted to the relation J(c)(T) similar to (1 - T/T-c)(n) with n = 1.9 and discussed in terms of thermally activated flux flow (TAFF). However, J(c)(B) results were explained based on the weak links, grain boundaries, and Josephson junctions in which it shows a power law behaviour J(c)(B) similar to B-0.5. At relatively low temperatures and magnetic fields, the critical current density was increased with a gamma-irradiation up to a dose of 100 MR. (C) 2000 Elsevier Science B.V. All rights reserved.