Differences in basal and ethanol-induced levels of opioid peptides in Wistar rats from five different suppliers

► Differences in opioid levels between Wistar rats from five different suppliers. ► Met-enkephalin-Arg6Phe7 and dynorphin B levels differed in several brain areas. ► Basal levels and levels after ethanol intake were supplier dependent. ► Areas where differences were found have all been implicated in alcohol addiction. ► The results have implications for study comparisons in preclinical opioid research.