The degradation rates of acetylcholine receptors (AchRs) were evaluated at the neuromuscular junction during and just after reinnervation of denervated muscles. When mouse sternomastoid muscles are denervated by multiple nerve crush, reinnervation begins 2-4 days later and is complete by day 7-9 after the last crush. In fully innervated muscles, the AChR degradation rate is stable and slow (t1/2 approximately 10 days), whereas after denervation the newly inserted receptors degrade rapidly (t1/2 approximately 1.2 days). The composite profile of degradation, which a mixture of the stable and the rapid receptors would give, is not observed during reinnervation. Instead, the receptors inserted between 2.5 and 7.5 days after the last crush all have an intermediate degradation rate of t1/2 approximately 3.7 days with standard error +/- 0.3 days. The total receptor site density at the endplate was evaluated during denervation and during reinnervation. As predicted theoretically, the site density increased substantially, but temporarily, after denervation. An analogous deleterious substantial decrease in density would be expected during reinnervation, without the intermediate receptor. This decrease is not observed, however, because of a large insertion rate at intermediate times (3000 +/- 700 receptor complexes per micro m2 per day). The endplate density of receptors thus remains relatively constant.