In the present study we have analyzed the characteristics and distribution of the mu-opioid receptor(MOR) by raising anti-peptide antisera to the C-terminal peptide of MOR. The antisera against MOR was produced in New Zealand White rabbit against 15 residue corresponding to amino acids, 384-398of the cloned rat MOR. The antigenic peptide was synthesized using an Applied Biosystems 432 solid-phase peptide aynthesizer. The specificity and identification fo the antisera were tested by analysisi fo transfected cells, epitope mapping and immunohistochemical method. COS-7 cells electroporated with MOR cDNA were used to evaluate the characteristics and subcellular distribution of MOR.MOR immunoreactivity was prodominent in the plasmalemma and subcellular compartments such as encoplasmicreticulum, Golgi apparatus and vesicle like structure. Furthermore, both tissue sections and transfected cell lines could be immunostained with these antisera and the immunoreactivity ws abolished when anti-MOR sera were preincubated with the peptide against which they wer raised. Based on epitope mapping analysis, all antisera appeared to have a similar epitope, which ws detemined to be within the last amino acid,391-398. Moreover, immunohistochemistry showed that MOR immunoreactivity was ovserved in many brain areas including cerebral cortex, striatum, hippocampus, locus coeruleus and the superficial laminae of the dorsal horn. These stained spinal cord and brain aras showed themirrored pattern observed in autoradiographic studies of mu-opioid binding as well as a pattern similar to that seen by in situ hybridization for MOR. Thus, several lines of evidence support the conclusion that the antisera produced in the present study most likely recognize mu-opioid receptor. These results suggest that MOR antisera may be utilized as useful tool to analyze the physiological and pharmacological studies for mu-opioid receptor in the future