We find that when extrajunctional regions of denervated soleus muscles in adult rats are transfected with cDNA encoding rat agrin isoform Y4Z8, which is normally secreted by motor neurons at adult neuromuscular junctions, the myofibers express and secrete the neural agrin. Muscle fibers in the vicinity of transfection form at their surface specialized areas having extracellular, plasma membrane, and cytoplasmic protein aggregates, narrow and deep plasma membrane infoldings, and an accumulation of myonuclei, all of which are characteristic of the postsynaptic apparatus at neuromuscular junctions. We conclude that at ectopic neuromuscular junctions that form in the extrajunctional region of denervated adult soleus muscles after implantation of a foreign nerve, a single neural-derived factor, agrin, is sufficient not only to cause protein aggregation in the early stages of postsynaptic apparatus formation, as predicted by the agrin hypothesis, but also to bring about changes in conformation of the muscle fiber surface and distribution of organelles which appear as the apparatus reaches maturity.