The myelin and lymphocyte protein MAL is a small proteolipid of 17 kDa and is expressed by oligodendrocytes and Schwann cells. We have characterized the embryonic and postnatal expression of MAL in the rat nervous system by in situ hybridization, immunocytochemistry, and western blotting and compared it with that of other myelin constituents. In the CNS, MAL is expressed during late steps of myelination: MAL protein appears approximately 3-5 days later than myelin basic protein and proteolipid protein. In contrast, in the PNS, MAL transcript and protein expression is detected prior to the onset of myelination, as early as embryonic day 17. Our results demonstrate that MAL is differentially expressed in oligodendrocytes and Schwann cells, likely reflecting different functions of the MAL proteolipid: (1) The late expression of MAL protein in the CNS points to a role in the final steps of myelin sheath formation, such as stabilization of the compacted myelin membranes. (2) The early expression of MAL protein in immature Schwann cells suggests an important role of MAL in the terminal differentiation step of the Schwann cell lineage and in the onset of peripheral myelination.