The tight link between synaptic activity and Arc transcription led to widespread use of Arc RNA as a means of determining what neurons are activated in response to various learning paradigms. Arc RNA appears in the nucleus within a few minutes of neuronal activation and is predominately localized to the cytoplasm by 30 min. In situ hybridization, therefore, reveals the history of the activity of individual neurons as well as the network of neurons activated during the formation of a new memory.