We carried out in situ resistivity measurements on macroscopic oriented ropes of single wall carbon nanotubes in a transmission electron microscope. We have found a minimum in the resistivity as a function of irradiation dose. This minimum is interpreted as a result of a twofold effect of the irradiation: the domination of covalent bond formation between tubes in a bundle due to broken bonds in the tube walls and the amorphization of the sample at high dose. Despite this improvement, the temperature dependence of the resistivity remains hopping-like. (C) 2003 American Institute of Physics.