en Behavioral Neuroscience Behavioral Neuroscience Original Original <p style="text-align: justify;"><strong>Introduction:</strong> This study investigated the effect of the environmental enrichment during adolescence on morphine-induced Conditioned Place Preference (CPP) and locomotor sensitization in maternally separated male and female rat pups.<br> <strong>Methods:</strong> Male Wistar rats were allowed to mate with female virgin Wistar rats. Pups were separated from them 3 hours per day during 2&ndash;14 days postnatal. All pups were weaned at 21 Postnatal Day (PND) and reared in standard environment or enriched environment from 21 to 50 PND with litter-mates of the same sex. The CPP and behavioral sensitization to morphine were assessed by an unbiased place conditioning paradigm and open filed method.<br> <strong>Results:</strong> The results showed that the maternal separation enhanced morphine-induced CPP in both sexes, locomotor sensitization in male pups and tolerance to morphine-induced motor activity in female pups during adolescence. While, male and female pups reared in an EE exhibited a decrease in morphine-induced CPP, locomotor sensitization and tolerance induced by maternal separation compared to their control pups.<br> <strong>Conclusion:</strong> Access to enriched environment during adolescence may have a protective effect against morphine-induced reward, locomotor sensitization and tolerance in adolescent male and female rats following maternal separation.</p> Maternal separation, Enriched environment, Conditioned place preference, Morphine, Behavioral sensitization 241 250 http://bcn.iums.ac.ir/browse.php?a_code=A-10-886-2&slc_lang=en&sid=1 Soheyla Khalaji shahzadkhalaji@yahoo.com 13700319475328460016355 13700319475328460016355 No Department of Psychology, Faculty of Psychology and Educational Sciences, Semnan University, Semnan, Iran. Imanollah Bigdeli bigdeli9@gmail.com 13700319475328460016356 13700319475328460016356 No Department of Psychology, Faculty of Educational Sciences and Psychology, Ferdowsi University of Mashhad, Mashhad, Iran. Raheb Ghorbani ghorbani.raheb93@gmail.com 13700319475328460016357 13700319475328460016357 No Social Determinates of Health Research Center, Semnan University of Medical Sciences, Semnan, Iran. Hossein Miladi-Gorji miladi331@yahoo.com 13700319475328460016358 13700319475328460016358 Yes Laboratory of Animal Addiction Models, Physiology Research Center, Semnan University of Medical Sciences, Semnan, Iran.