We have studied end-linked polystyrene networks containing labeled (deuterated) free chains subjected to an uniaxial deformation using small-angle neutron scattering(SANS). The networks are made by end-linking protonated polystyrene precursor chains of a narrow size distribution. As for statistical networks investigated earlier, the isointensity lines on the bidimensional detector have the double-winged shape known as "butterfly patterns". They appear as a figure "8" at small values of the scattering vector, q, oriented along the stretching direction, corresponding to a larger scattering in the parallel than in the perpendicular direction even at q-->0. We relate the existence of butterfly patterns to preexisting heterogeneities in the cross-linking density, which cause spatial fluctuations of the free chain concentration. Such heterogeneities are therefore present even in end-linked networks. They can be attributed to imperfect chemistry or to more physical origins : heterogeneity in the spatial distribution of entanglement or the presence of large loops.