The evolution of GaN luminescence under electron beam injection has been studied by means of in situ cathodoluminescence experiments on various epitaxial lateral overgrown samples. It is shown that the ultraviolet (UV) peak of undoped materials experiences a decrease of its intensity as well as a noticeable redshift, while the other extrinsic peaks only experience an intensity decrease. However, in Mg doped materials the UV peak intensity decrease is followed by an increase of its intensity which can even reach larger values than the initial one. We suggest that all these features are self-consistently explained by the occurrence of strain relaxation resulting from the beam enhanced diffusion of vacancies from the free surface, and from the coalescence boundaries towards the bulk. (C) 2001 American Institute of Physics.