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In rats, contact with pups at parturition establishes a form of maternal memory that enables female rats to respond rapidly to pups in the future. Treatment of pregnant female rats with the long-lasting μ opioid receptor antagonist, β-funaltrexamine (β-FNA), prior to parturition interfered with the establishment of maternal memory. Similar treatment 3 hr postpartum resulted in disrupted retention of maternal memory that appeared nonspecific, with both drug- and vehicle-treated rats displaying a deficit. However, infusion of the opioid antagonist 24 hr postpartum had no effect on the retention of maternal memory tested 7 days later. These findings indicate that the establishment of maternal memory is mediated by endogenous opioid activity around the time of parturition.