Assessment of the aversive effects of peripheral mu opioid receptor agonism in Fischer 344 and Lewis rats

► Morphine induces differential CTAs in F344 and LEW rats (F > L). ► Morphine's differential effects may be mediated centrally or peripherally. ► The peripheral mu agonist loperamide induced similar aversions in the two strains. ► Differences in loperamide and morphine's effects in F344/LEW rats were discussed.