Opioid-binding cell adhesion molecule (OBCAM), a neuron-specific protein, consists of three immunoglobulin (Ig)-like domains anchored to the membrane through a glycosylphosphatidylinositol (GPI)-tail. OBCAM has been presumed to play a role as a cell adhesion/recognition molecule, but its function has not been fully elucidated. We investigated the developmental expression of OBCAM in rat brain by using a monoclonal anti-OBCAM peptide antibody (OBC53). OBCAM was clearly detectable on embryonic day 16 (E16) as assessed by immunoblotting. The expression level increased by the second postnatal week and was maintained at a constant level until week 17. During the early developmental period OBCAM was found to be expressed on postmitotic neurons and to be strongly expressed in at the fiber tracts containing expanding axons, in contrast to the adult brain, in which OBCAM is principally expressed in the gray matter. These findings suggest that the function of OBCAM involves axonal outgrowth.