The development of acetylcholine (ACh) receptor aggregates at the neuromuscular junction was studied in rat sternomastoid muscles. The first junctional clusters of ACh receptors were loose aggregates of small receptor patches (15 1/2 to 16 1/2 days of gestation). These clusters coalesced to more compact but simple plaques (18 days of gestation to 3 days postnatal). During the first 2 weeks postnatal several changes occurred: the receptor plaque was modified to the adult junctional receptor distribution, multiple innervation was eliminated, and extrajunctional ACh receptors were lost. At the time of birth the receptors in the junctional receptor plaque were already degraded more slowly than were extrajunctional receptors. The junction increased in length most rapidly during the period when the muscle increased most rapidly in mass, from 15 to 100 days postnatal. It is concluded that the junction goes through several stages during development.