From an NG108-15 cDNA eucaryotic expression library, we recently identified and characterized a delta-opioid receptor clone (DOR-1) with the anticipated affinity and selectivity for ligands binding delta-opioid receptors(l). Using a similar strategy an essentially identical clone was isolated by Keiffer et al. (2) from the same cell line. Based upon Southern analysis DOR-1 is a murine clone and the NG108-15 cells (a rat/mouse hybrid line) have lost the genetic material encoding the rat DOR-1 sequence. Here we present some insights into the gene structure and regulation of DOR-1 in NG108-15 cells. Finally, we report on the generation of antisera to DOR-1 and our findings concerning the anatomical distribution of this delta opioid receptor in rodent brain.