Abstract Morphine is a widely-prescribed and potent opioid analgesic, but in recent years its non-medical use has been on the rise and has contributed to the increased incidence of opioid use disorder. Repeated exposure to morphine and other drugs of abuse leads to lasting learned associations between the rewarding properties of the drug and the environment of administration. Therefore even in abstinence, re-exposure to the context is a risk factor for relapse, increasing withdrawal symptoms and drug cravings. While the neural circuits mediating drug-context associations and drug seeking behavior have been studied heavily, the specific underlying mechanisms of these associations remain poorly understood. In the proposed studies, we aim to evaluate the hypothesis that epigenetic alterations in the methylation of genes related to neuronal connectivity and excitability during repeated morphine-context pairings provide a mechanism for the stable recruitment of a small, specific population of neurons in the ventral hippocampus to the ‘engram” storing the memory of morphine-context associations. Switching the methylation status of these genes alters gene expression, which leads to increased excitability and connectivity with pre- and postsynaptic circuit cortical and limbic partners to enhance the morphine-context association and create a lasting memory. By characterizing the changes in methylation in the recruited hippocampal neuronal ensemble and evaluating the consequent effects on neuronal function and circuit plasticity, our experiments may provide a new framework for the study of the mechanisms of opioid addiction and contribute to more effective treatments of substance use disorder.