The efficacy of synaptic transmission depends on the maintenance of a high density of neurotransmitter receptors and their associated scaffold proteins in the postsynaptic membrane. While the dynamics of receptors has been extensively studied, the dynamics of the intracellular scaffold proteins that make up the postsynaptic density are largely unknown in vivo. Here, we focused on the dynamics of rapsyn, a protein required for the clustering and maintenance of acetylcholine receptor (AChR) density at postsynaptic sites. Using time-lapse imaging, we demonstrated that rapsyn is remarkably dynamic compared to AChRs at functional synapses, turning over 4–6 times more rapidly than AChRs. In addition we found that the rapid turnover of rapsyn is insensitive to alterations in synaptic activity, whereas AChR turnover is profoundly affected, illustrating that rapsyn and receptor dynamics are controlled by distinct mechanisms. These data indicate that individual postsynaptic components are in permanent exchange despite the overall stability of synaptic structure, which may play a role in synaptic plasticity.