The behaviour of a cetylpyridinium chloride/sodium salicylate wormlike micellar system undergoing capillary breakup is investigated experimentally. The transient elongational flow induced in a thinning thread is used to study the apparent viscosity and effective relaxation time of the micellar network in extension. The sensitivity of the material response to initial conditions and deformation history is explored by varying the endplate diameter of the capillary breakup rheometer and the initial step strain imposed on the sample. Local regimes in which the extensional viscosity shows constant (Newtonian), weak extensional thinning and strong extensional thickening are observed depending on the accumulated strain and strain rate. For this concentrated micellar network, the relaxation time determined in extension was found to be independent of the initial configuration and in close agreement with the value measured in shear flow.