125I-[D-Ala2,p-N3-Phe4-Met5]Enkephalin was used to investigate the opioid receptor from brain membranes of adult (6 months old) and fetal (16 days gestation) mice. The photolabeled membranes were analyzed by sodium dodecyl sulfate gel electrophoresis. A 46,000-Da protein was specifically photo-labeled in the adult mouse brain membranes. The photolabeling of this protein was inhibited in the presence of [D-Ala2,Met5]enkephalin and a peptide ligand specific for the mu-opioid receptor. No specific labeling of any protein was detected in the brain membranes of the fetal mice. This apparent lack of the opioid receptor in the fetal mouse brain suggests that the appearance of the opioid receptor in the adult mouse brain is subject to developmental regulation.