AbstractAMPA receptor (AMPAR) mobility within synapses has been extensively studiedin vitro. However, whether similar mobility properties apply to AMPARsin vivohas yet to be determined. Here, we use two-photon fluorescence recovery after photobleaching (FRAP) to study AMPAR mobility within individual dendritic spines in live animals using an overexpression vector. We demonstrate the existence of mobile and immobile fractions of AMPARs across multiple cortical regions and layers. Additionally, we find that AMPAR mobility can be alteredin vivoin response to administration of corticosterone, a condition that mimics exposure to stress.