Distribution and characteristics of met5-enkephalin-arg6-phe7 (YGGFMRF) in the rat brain was studied by radioimmunoassay. The YGGFMRF is unevenly distributed in the brain with the highest content in striatum and hypothalamus and the lowest in the hippocampus and cerebellum. The distribution of YGGFMRF is very similar to that of met5-enkephalin. However, the content of YGGFMRF is smaller than that of met5-enkephalin in every region studied. The YGGFMRF can be released from striatal slices by depolarizing concentration of KCl in a Ca++ dependent manner raising the possibility of a neuroregulatory role for YGGFMRF. The YGGFMRF immunoreactivity in striatum and hypothalamus is composed mainly of authentic YGGFMRF. However, multiple forms of immunoreactive material, which include high molecular weight forms, exist in some brain regions. Further characterization of high molecular weight YGGFMRF is in progress. The uneven distribution of YGGFMRF suggests that this endogenous opioid peptide may participate in neuronal transmission in brain function. The possible interaction between dopamine and YGGFMR in striatum will be studied using specific drugs.